diff --git a/Makefile b/Makefile
index eb1267b..40c9439 100644
--- a/Makefile
+++ b/Makefile
@@ -20,7 +20,8 @@ BUILD_FILES = src/dura.tab.c \
               src/lex.parser1.c \
               src/lex.parser2.c \
               src/lex.parser3.c \
-              src/lex.parser4.c
+              src/lex.parser4.c \
+              src/lex.parser5.c
 
 C_FILES := src/asmsx.c src/labels.c
 
diff --git a/src/dura.y b/src/dura.y
index 1bf504b..5d11ecf 100644
--- a/src/dura.y
+++ b/src/dura.y
@@ -32,6 +32,7 @@ int preprocessor1(char *);	/* defined in parser1.l */
 int preprocessor2();		/* defined in parser2.l */
 int preprocessor3(int);		/* defined in parser3.l */
 int preprocessor4();		/* defined in parser4.l */
+int preprocessor5(const char*);		/* defined in parser5.l */
 
 /* forward function declarations to address GCC -Wimplicit-function-declaration warnings */
 void yyerror(char *);
@@ -4690,6 +4691,7 @@ int main(int argc, char *argv[]) {
   preprocessor3(zilog);     // Zilog
   preprocessor4();          //Macros
   snprintf(fname_p2, PATH_MAX - 1, "~tmppre.%i", preprocessor2()); // REPT
+  snprintf(fname_p2, PATH_MAX - 1, "~tmppre.%i", preprocessor5(fname_p2)); // Conditional compilation (IFDEF/ELSE/ENDIF)
   printf("Assembling source file %s\n", fname_asm);
 
   conditional[0] = 1;
diff --git a/src/parser5.l b/src/parser5.l
new file mode 100644
index 0000000..4b3d047
--- /dev/null
+++ b/src/parser5.l
@@ -0,0 +1,335 @@
+/*
+  PARSER-5
+  (c) 2025 asMSX team
+
+  Functions:
+    1.- Process IFDEF/ELSE/ENDIF conditional compilation directives
+    2.- Strip out code in false conditional blocks
+    3.- Track label definitions for IFDEF support
+    4.- Pass through IF/ELSE/ENDIF directives to main parser (expressions evaluated there)
+*/
+
+%{
+#include "asmsx.h"
+
+#define MAX_CONDITIONALS 16
+#define MAX_SYMBOLS 8192
+
+// Type of conditional
+#define COND_IF 1
+#define COND_IFDEF 2
+
+static FILE *p5_output;
+static char *p5_name;
+static int p5_lines;
+
+// Conditional compilation state
+typedef struct {
+    int type;      // COND_IF or COND_IFDEF
+    int enabled;   // Is this block enabled?
+} conditional_state;
+
+static conditional_state conditional_stack[MAX_CONDITIONALS];
+static int conditional_level = 0;
+
+// Symbol tracking for IFDEF
+static char *defined_symbols[MAX_SYMBOLS];
+static int num_symbols = 0;
+
+int prompt_error5(int, char*, int);
+int is_symbol_defined(char* name);
+void define_symbol(char* name);
+
+static inline int is_output_enabled() {
+    if (conditional_level == 0) return 1;
+    return conditional_stack[conditional_level].enabled;
+}
+%}
+
+%option noinput nounput noyywrap
+
+%%
+
+"#"file[ \t]*\"[a-zA-Z_][a-zA-Z0-9_]*"."[a-zA-Z_][a-zA-Z0-9_]*\"\n {
+    // Always output file markers
+    fprintf(p5_output, "%s", yytext);
+
+    // Save filename
+    char *dup = strdup(yytext);
+    char *p5_tmpstr = strtok(dup, "\"");
+    p5_tmpstr = strtok(NULL, "\"");
+    if (p5_tmpstr) {
+        p5_name[0] = '\0';
+        safe_strcat(p5_name, p5_tmpstr, PATH_MAX, p5_name, p5_lines);
+    }
+    free(dup);
+}
+
+"#"line[ \t]*[0-9]+\n  {
+    // Always output line markers
+    fprintf(p5_output, "%s", yytext);
+    p5_lines = atoi(&yytext[5]);
+}
+
+"."?[iI][fF][dD][eE][fF][ \t]+[a-zA-Z_][a-zA-Z0-9_]*[ \t]*\n {
+    // Extract symbol name
+    char *line_copy = strdup(yytext);
+    char *name_start = line_copy;
+    if (name_start[0] == '.') name_start++;
+    name_start += 5; // skip "ifdef"
+    while (*name_start == ' ' || *name_start == '\t') name_start++;
+    
+    // Remove trailing whitespace and newline
+    char *end = name_start + strlen(name_start) - 1;
+    while (end > name_start && (*end == ' ' || *end == '\t' || *end == '\n')) {
+        *end = '\0';
+        end--;
+    }
+    
+    if (conditional_level >= MAX_CONDITIONALS - 1) {
+        free(line_copy);
+        prompt_error5(1, p5_name, p5_lines);
+    }
+    
+    conditional_level++;
+    conditional_stack[conditional_level].type = COND_IFDEF;
+    int parent_enabled = (conditional_level == 1) ? 1 : conditional_stack[conditional_level - 1].enabled;
+    
+    if (parent_enabled && is_symbol_defined(name_start)) {
+        conditional_stack[conditional_level].enabled = 1;
+    } else {
+        conditional_stack[conditional_level].enabled = 0;
+    }
+    
+    free(line_copy);
+    // Don't output the IFDEF directive itself
+}
+
+"."?[iI][fF][ \t]+ {
+    // For IF with expression, pass through to main parser
+    // But track it so we know to pass through corresponding ELSE/ENDIF
+    if (conditional_level >= MAX_CONDITIONALS - 1) {
+        prompt_error5(1, p5_name, p5_lines);
+    }
+    
+    conditional_level++;
+    conditional_stack[conditional_level].type = COND_IF;
+    int parent_enabled = (conditional_level == 1) ? 1 : conditional_stack[conditional_level - 1].enabled;
+    conditional_stack[conditional_level].enabled = parent_enabled;
+    
+    // Pass through to main parser if parent is enabled
+    if (is_output_enabled()) {
+        fprintf(p5_output, "%s", yytext);
+    }
+}
+
+"."?[eE][lL][sS][eE][ \t]*\n {
+    if (conditional_level == 0) {
+        prompt_error5(2, p5_name, p5_lines);
+    }
+    
+    if (conditional_stack[conditional_level].type == COND_IFDEF) {
+        // Handle ELSE for IFDEF
+        int parent_enabled = (conditional_level == 1) ? 1 : conditional_stack[conditional_level - 1].enabled;
+        int was_enabled = conditional_stack[conditional_level].enabled;
+        conditional_stack[conditional_level].enabled = parent_enabled && !was_enabled;
+        // Don't output the ELSE directive itself
+    } else {
+        // Pass through ELSE for IF
+        if (is_output_enabled()) {
+            fprintf(p5_output, "%s", yytext);
+        }
+    }
+}
+
+"."?[eE][nN][dD][iI][fF][ \t]*\n {
+    if (conditional_level == 0) {
+        prompt_error5(3, p5_name, p5_lines);
+    }
+    
+    if (conditional_stack[conditional_level].type == COND_IFDEF) {
+        // Handle ENDIF for IFDEF
+        conditional_level--;
+        // Don't output the ENDIF directive itself
+    } else {
+        // Pass through ENDIF for IF
+        if (is_output_enabled()) {
+            fprintf(p5_output, "%s", yytext);
+        }
+        conditional_level--;
+    }
+}
+
+[a-zA-Z_][a-zA-Z0-9_]*:[ \t]*\n {
+    // Label definition - track it
+    char *label = strdup(yytext);
+    char *colon = strchr(label, ':');
+    if (colon) *colon = '\0';
+    
+    define_symbol(label);
+    free(label);
+    
+    if (is_output_enabled()) {
+        fprintf(p5_output, "%s", yytext);
+    }
+}
+
+[a-zA-Z_][a-zA-Z0-9_]*[ \t]+[eE][qQ][uU][ \t]+ {
+    // EQU definition - track the symbol name
+    char *label = strdup(yytext);
+    char *space = label;
+    while (*space && *space != ' ' && *space != '\t') space++;
+    *space = '\0';
+    
+    define_symbol(label);
+    free(label);
+    
+    if (is_output_enabled()) {
+        fprintf(p5_output, "%s", yytext);
+    }
+}
+
+[a-zA-Z_][a-zA-Z0-9_]*[ \t]*"="[ \t]* {
+    // Assignment definition - track the symbol name
+    // Handles both "VAR = value" and "VAR=value"
+    char *label = strdup(yytext);
+    char *eq = strchr(label, '=');
+    if (eq) *eq = '\0';
+    // Trim trailing spaces
+    char *end = label + strlen(label) - 1;
+    while (end > label && (*end == ' ' || *end == '\t')) {
+        *end = '\0';
+        end--;
+    }
+    
+    define_symbol(label);
+    free(label);
+    
+    if (is_output_enabled()) {
+        fprintf(p5_output, "%s", yytext);
+    }
+}
+
+\n  {
+    if (is_output_enabled()) {
+        fprintf(p5_output, "%s", yytext);
+    }
+}
+
+.   {
+    if (is_output_enabled()) {
+        fprintf(p5_output, "%s", yytext);
+    }
+}
+
+%%
+
+int prompt_error5(int c, char* filename, int line) {
+    fprintf(stderr, "%s, line %d: ", filename, line);
+    switch (c) {
+        case 1:
+            fprintf(stderr, "Too many nested conditional levels\n");
+            break;
+        case 2:
+            fprintf(stderr, "ELSE without IF/IFDEF\n");
+            break;
+        case 3:
+            fprintf(stderr, "ENDIF without IF/IFDEF\n");
+            break;
+        default:
+            fprintf(stderr, "Unknown error in prompt_error5()\n");
+    }
+    exit(c);
+    return c;
+}
+
+int is_symbol_defined(char* name) {
+    for (int i = 0; i < num_symbols; i++) {
+        if (strcmp(defined_symbols[i], name) == 0) {
+            return 1;
+        }
+    }
+    return 0;
+}
+
+void define_symbol(char* name) {
+    if (num_symbols >= MAX_SYMBOLS) {
+        fprintf(stderr, "WARNING: Symbol table overflow in preprocessor5, symbol '%s' not tracked for IFDEF\n", name);
+        return;
+    }
+    
+    // Check if already defined
+    if (is_symbol_defined(name)) {
+        return;
+    }
+    
+    defined_symbols[num_symbols] = strdup(name);
+    num_symbols++;
+}
+
+int preprocessor5(const char* input_filename) {
+    FILE *input;
+
+    // Initialize
+    conditional_level = 0;
+    num_symbols = 0;
+    
+    // Add ASMSX as a predefined symbol
+    define_symbol("ASMSX");
+
+    p5_name = malloc(PATH_MAX);
+    if (!p5_name) {
+        fprintf(stderr, "Fatal: cannot allocate memory for p5_name\n");
+        exit(1);
+    }
+    p5_name[0] = 0;
+    p5_lines = 0;
+
+    if ((input = fopen(input_filename, "r")) == NULL) {
+        fprintf(stderr, "Fatal: cannot process file %s\n", input_filename);
+        exit(1);
+    }
+
+    yyin = input;
+    
+    // Determine output filename by incrementing the input number
+    char output_filename[PATH_MAX];
+    int file_num;
+    if (sscanf(input_filename, "~tmppre.%d", &file_num) == 1) {
+        snprintf(output_filename, PATH_MAX, "~tmppre.%d", file_num + 1);
+    } else {
+        // Fallback
+        snprintf(output_filename, PATH_MAX, "~tmppre.3");
+    }
+    
+    p5_output = fopen(output_filename, "w");
+
+    if (p5_output == NULL) {
+        fprintf(stderr, "ERROR: cannot create file %s in %s\n", output_filename, __func__);
+        exit(1);
+    }
+
+    yylex();
+
+    // Check for unmatched IFDEF conditionals (IF conditionals checked by main parser)
+    if (conditional_level != 0) {
+        for (int i = 1; i <= conditional_level; i++) {
+            if (conditional_stack[i].type == COND_IFDEF) {
+                fprintf(stderr, "ERROR: Unmatched IFDEF directive\n");
+                exit(45);  // Error code 45: Unmatched IF/IFDEF directive
+            }
+        }
+    }
+
+    fclose(input);
+    fclose(p5_output);
+
+    free(p5_name);
+    
+    // Free symbol memory
+    for (int i = 0; i < num_symbols; i++) {
+        free(defined_symbols[i]);
+    }
+
+    return file_num + 1;
+}
diff --git a/test/features/bugs.feature b/test/features/bugs.feature
index 0a066b6..796228b 100644
--- a/test/features/bugs.feature
+++ b/test/features/bugs.feature
@@ -160,6 +160,50 @@ Feature: Fixing issues
     And sym does not contain If it's a label, it may not be defined.
     And build has no warnings
 
+  Scenario: IFDEF with invalid syntax in false block should not cause error
+    Given I write the code to test.asm
+      """
+      .BIOS
+      .BIOSVARS
+      .FILENAME "TESTIFD.ROM"
+      
+      LABELEXIST:
+      
+      ; Case 1: The Label does not exist. The ELSE must be processed.
+      IFDEF THISLABELNOTEXIST
+          CONSTANT1:	EQU $01
+          CONSTANT2:	EQU $02
+      ELSE
+      CONSTANT1	EQU $01
+      CONSTANT2	EQU $02
+      ENDIF
+      
+      ; Case 2: The Label exist. The ELSE does not have to be processed.
+      IFDEF LABELEXIST
+      CONSTANT3	EQU $03
+      CONSTANT4	EQU $04
+      ELSE
+          CONSTANT3:	EQU $03
+          CONSTANT4:	EQU $04
+      ENDIF
+      
+      .PAGE	1
+      .ROM
+      
+      MAIN:
+        DI
+        EI
+        ret
+      """
+    When I build test.asm
+    Then file TESTIFD.ROM.rom exists
+    And sym contains CONSTANT1
+    And sym contains CONSTANT2
+    And sym contains CONSTANT3
+    And sym contains CONSTANT4
+    And sym contains LABELEXIST
+    And build has no warnings
+
   @wip
   Scenario: Issue #88 Local label semantics
     Given I write the code to test.asm