Access does not really like null values. A null value is not equal to another null value.
The key is to use the Nz function, which converts a Null value to a value of your choosing.
So, try something like:
Code:
IIF(Nz([Table1].[Field1],"Empty")=Nz([Table2].[Field1],"Empty"),True,False)
By the way, I think you can still post to that other thread. It isn't locked out or anything. The "Solved" is just a flag to let people know at a glance that it has been solved.
But you can still reply to it, and I will see those.