Peter,
Thanks for the quick answer. I think I get your meaning. I had been using Access' 'filter by' feature to determine the 'correct' number of rows.

Checking for overlap, as you mention - shows that the results from ="" and Is Null are indeed independent data sets (I should have seen that - I still wouldn't have known why though). I guess not all blank fields are created equal!

Thank you very much. The data has now been properly cleaned using:

UPDATE TABLE.FIELD = Null
WHERE (((TABLE.FIELD)=""));

-Zeke