The hydrogen isotopic composition (δD) of long-chain alkenones derived from haptophyte algae has recently been applied as paleo sea surface salinity proxy, but this application has been complicated by reported differences in δD of individual C37 alkenones of up to 45‰. Here we show that this difference in D/H ratios is the result of the desaturation of the C37:2 alkenone to form the C37:3 alkenone and related to the relative abundance of the individual alkenones, and, therefore, the UK'37. For the purpose of reconstructing paleo sea surface salinities it is recommended to determine the δD of combined rather than individual C37 alkenones. © 2012 Elsevier Ltd.