The EI mass spectra of 3,4-disubstituted-1,2,4-oxadiazin-5-ones 1-6 and -thiones 7,8 and 3,5-disubstituted 1,2,4-oxadiazin-6-ones 9,10 were recorded and their fragmentation pathways solved and compared with each other. The fragmentation routes of 5-ones and 5-thiones do not differ very much from each other but compounds 9 and 10 behave differently as could be expected based on their lactone type structures. Only compounds 4-6 exhibit a loss of CO and compound 7 a loss of NO. The loss of a benzyl group dominates the behaviour of compounds 1 and 2 which showed only few additional fragmentations. Some earlier data on some 3,4-disubstituted-1,2,4-diazin-5-ones 11-13 and -thiones 14,15 have been reanalyzed and discussed in further detail.