It's working precisely as it should.

Let's say you have ItemIDs 35 and 45 in both tables.

When you create this "Cartesian Join" query, with no join line, Access
will pair every single record in tbl_Items with every record in
tbl_BaseItems - that is, if you have 1000 records in tbl_Items, and
2000 in tbl_BaseItems, you'll get all 2,000,000 possible combinations.

When you look at the combined record comparing tbl_Items ItemID 35
with tbl_BaseItems ItemID 35 the record will be correctly rejected -
but down that long list you'll be comparing tbl-Items 35 with
tbl_BaseItems 45; your inequality criterion will be true, so you'll
see Item 35.

The correct SQL is a "frustrated outer join":

SELECT tbl_Items.ItemID
FROM tbl_Items LEFT JOIN tblBase_Items
ON tblBase_Items.ItemID = tbl_Items.ItemID
WHERE tblBase_Items.ItemID IS NULL;

This works by finding all records in tbl_Items; joining each with its
corresponding record in tblBase_Items, if there is one; and excluding
all those records which DO have a match. The ones which don't will
have a NULL returned for tblBase_Items.ItemID.

                 John W. Vinson[MVP]