2-Bis-(chlorodifluoromethyl)-4-substituted-1,3-oxazolidin-5-ones, a new type of alpha-amino acid derivative for gas chromatographic separation, have been studied by low resolution mass spectrometry. These derivatives are obtained by reacting alpha-amino acids with dichlorotetrafluoroacetone. Their structure has been established or confirmed for most protein amino acids and several non-protein alpha-amino acids. The mechanisms responsible for the mass spectral pattern have been rationalized. An interesting feature of this derivatization procedure is that it distinguishes aspartic and glutamic acid from the respective amides. The structure of asparagine and glutamine derivatives has been established. A survey of oxazolidinone mass spectra has provided a list of diagnostically useful ions. Each amino acid can be identified by one or two of its most abundant fragments.