The mass spectra of eight [gamma]-lactones and eight [delta] -lactones have been determined. Both homologous series give rise to characteristic peaks at m/e = 85 and 99 for [gamma]- and [delta] -lactones, respectively. The abundance of the molecular ion is in general very low and it is difficult to determine the molecular size of an unknown lactone on the basis of the mass spectra alone. The best criterion for this is the gas chromatographic retention time. Gas chromatographic retention times in a poly(propylene glycol) column at 175[degree]C for the [gamma]- and [delta]-lactones investigated are given.