even if A is an integer multiple of B, (A MOD B) = B if B is too small. A MOD B =0 would be correct. i am not sure what the threshold value for this behavior is, but it is between B = 0.2 and B = 0.25. this is for IDL 8.4 on the mac, so perhaps it has been corrected in later versions. i may not check back here for replies... i just wanted to bring this to people's attention in case it's helpful. here is some sample output showing the behavior (and the workaround on the last line): IDL> print, 100d mod 2d 0.0000000 IDL> print, 100d mod 1d 0.0000000 IDL> print, 100d mod .5d 0.0000000 IDL> print, 100d mod .25d 0.0000000 IDL> print, 100d mod .2d 0.20000000 IDL> print, ((100d * 10) mod (0.2d * 10)) / 10 0.0000000