The electron impact fragmentation of 1, 2-O-isopropylidene-α-D-glycero-tetros-3-ulofuranose has been studied with the help of deuterium and ¹⁸O labelling. The effect of substituting a hydrogen atom of C-4 for a CH₃˙ radical and that of replacing the carbonyl function with a cyanomethylenic substituent is also reported.