Testing if NSMutableArray contains a string object

What you’re doing should work fine. For example

NSArray *a = [NSArray arrayWithObjects:@"Foo", @"Bar", @"Baz", nil];
NSLog(@"At index %i", [a indexOfObject:@"Bar"]);

Correctly logs “At index 1” for me. Two possible foibles:

1. indexOfObject sends isEqual messages to do the comparison – you’ve not replaced this method in a category?
2. Make sure you’re testing against NSNotFound for failure to locate, and not (say) 0.