Previous work has suggested that a species effect may be present in diatom oxygen isotope ratios (δ18Odiatom). While predominantly attributed to be a size-related species effect, currently the precise mechanism remains unknown. Here, two size fractions of diatoms (38–75 µm and >100 µm) covering the last 200 ka are analysed for δ18O from ODP Site 882 in the North-West Pacific Ocean. Synchronous variations of up to 13‰ occur in both size fractions. However, large isotope offsets (mean = 2.02‰) exist between the two fractions with no relationship between their magnitude and the overlying palaeoenvironmental conditions. In contrast to earlier work from the same site, no one size fraction is constantly higher or lower in δ18O relative to the other. As such, the dominant mechanism is most likely separate to the size effect previously detected. In addition, no relationship exists between the magnitude of the offsets and the species composition of the two size fractions. The presence of these isotope offsets imposes significant constraints upon the future use of δ18Odiatom in palaeoceanographic reconstructions and reiterates the need to extract and analyse only species- and size-specific diatom samples. © Natural Environment Research Council (NERC) copyright 2008. Reproduced with the permission of NERC. Published by John Wiley & Sons, Ltd.