The mass spectrometric behaviour under electron ionization of several 3,4-(alkyl/aryl)-disubstituted 1,2,4-oxadiazole-5(4H)-ones (1-13) and 1,2,4-thiadiazole-5(4H)-thiones (14-17), and that of 3-aryl-5-alkyl- or arylthio-1,2,4-thiadiazoles (18-24), was studied. These five-membered rings split similarly to the corresponding 1,2,4-thiadiazole-5(4H)-ones, although substitution has also a clear effect on the routes of fragmentation and the magnitude of secondary processes. In particular, the fragmentation of 1,2,4-oxadiazole-5(4H)-ones (1-6), which do not bear aromatic substituents, was, in addition to the ring ruptures, fairly complicated. The other compounds fragmented more systematically and relatively few unpredictable fragmentations occurred. Copyright (C) 2004 John Wiley Sons, Ltd.