Theoretical calculations on the gas-phase thermal decomposition of dimethyldiazirine, diethyldiazirine and difluorodiazirine have been carried out using ab initio composite methods CBS-QB3 and G3, and DFT CAM-B3LYP, MPW1PW91, PBE1PBE and M062X. Reasonable agreement has been found with the experimental values by the G3 method. Two possible mechanisms were studied: Mechanism A consists the opening of the heterocyclic ring with hydrogen transfer to a nitrogen atom to form prop-1-en-2-yldiazene intermediate which later decomposes to nitrogen gas and the corresponding olefin. Mechanism B involves the extrusion of nitrogen molecule with the formation of carbene intermediate which subsequently yields the corresponding olefin. The results obtained from G3 calculations support the mechanism of a carbene intermediate. Population analysis and Wiberg's bond order show an interaction between the leaving nitrogen and the electron deficient carbon formed during the first step of the reaction.