The fragmentation behaviour of 3,4-disubstituted 1,2,4-oxadiazole-5(4H)-thiones under electron impact (EI) can be explained by postulating a predominant rearrangement to the corresponding 3,4-disubstituted 1,2,4-thiadiazole-5(4H)-ones before further fragmentation. This postulate is confirmed by a comparison with the EI spectra of the latter compounds obtained independently by thermal rearrangement since they do not undergo a reverse rearrangement under EI, Derivatives with a butyl group at C-3 or a propyl group at N-4 fragment mainly via the [M-C4H8](+.) or [M-C3H6](+.) ions, respectively, the decomposition of which resembles closely that of the diaryl- or dimethyl-substituted derivatives. Copyright (C) 1999 John Wiley & Sons, Ltd.