We illustrate offsets in seawater isotopic composition between the data sets presented in two recent studies and the LOCEAN seawater isotopic composition dataset, as well as other data for the same years and same regions. These comparisons are carried in the surface waters, for one in the North and South Atlantic, and for the other in the subtropical South-East Indian Ocean. The observed offsets between data sets which exceed 0.10 ‰ in δ18O and 0.50 ‰ in δ2H might in part reflect seasonal or spatial variability. However, they are rather systematic, so they likely originate, at least partially, from different instrumentations and protocols used to measure the water samples. They need to be adjusted in order to ultimately merge the different data sets. This highlights the need to actively share seawater isotopic composition samples dedicated to specific intercomparison of data produced in the different laboratories.