The thermal and electron-impact (EI) stabilities of eight 3,6-disubstituted-1,2,4,5-tetrazines were examined. Major EI peaks were also investigated by tandem mass techniques to construct fragmentation pathways. Tetrazine ring structures were not maintained under electron impact ionization nor under conditions of thermal decomposition. Under electron impact, the tetrazines examined shared the same initial fragmentation pathways: elimination of two of the nitrogen atoms as N-2 from the tetrazine ring and cleavage of the remaining N-N bond. This pathway was also applicable to thermal decomposition; however, the main route involved dissociation of the substituent group, in some cases assisted by proton transfer.