diff --git a/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/scanner/PreprocessorBugsTests.java b/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/scanner/PreprocessorBugsTests.java
index e6ed8b24c7d..58c8766127e 100644
--- a/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/scanner/PreprocessorBugsTests.java
+++ b/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/scanner/PreprocessorBugsTests.java
@@ -149,4 +149,20 @@ public class PreprocessorBugsTests extends PreprocessorTestsBase {
 		validateEOF();
 		validateProblemCount(0);
 	}
+
+	// #if if
+	// no
+	// #else
+	// yes
+	// #endif
+	// #if or
+	// no
+	// #endif
+	public void testKeywordsInConditionalExpression_Bug246369() throws Exception {
+		initializeScanner();
+		validateIdentifier("yes");
+		validateEOF();
+		validateProblemCount(1);
+		validateProblem(0, IProblem.SCANNER_EXPRESSION_SYNTAX_ERROR, null);
+	}
 }
diff --git a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/parser/scanner/ExpressionEvaluator.java b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/parser/scanner/ExpressionEvaluator.java
index 3cf380f0458..614c5281f8f 100644
--- a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/parser/scanner/ExpressionEvaluator.java
+++ b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/parser/scanner/ExpressionEvaluator.java
@@ -257,7 +257,27 @@ class ExpressionEvaluator {
         case IToken.tIDENTIFIER:
         	consume();
         	return 0;
+        // 16.1.4 alternate keywords are not replaced by a 0
+        case IToken.tAND:
+        case IToken.tOR:
+        case IToken.tBITOR:
+        case IToken.tBITORASSIGN:
+        case IToken.tXOR:
+        case IToken.tXORASSIGN:
+        case IToken.tAMPER:
+        case IToken.tAMPERASSIGN:
+        case IToken.tSTRING:
+        case IToken.tLSTRING:
+            throw new EvalException(IProblem.SCANNER_EXPRESSION_SYNTAX_ERROR, null); 
+        	
         default:
+            // 16.1.4 keywords are replaced by 0
+        	final char[] image= fTokens.getCharImage();
+        	if (image.length > 0) {
+        		final char c= image[0];
+        		if ((c>='a' && c<='z') || (c>='A' && c<='Z') || c == '_' || c=='$' || c=='@')
+        			return 0;
+        	}
             throw new EvalException(IProblem.SCANNER_EXPRESSION_SYNTAX_ERROR, null); 
         }
     }