Ab initio molecular orbital calculations have been used to study the decomposition of methyl azide (CH3N3), methanimine, and its isomers (CH3N) in both lowest lying singlet and triplet states. Geometries were optimized using UMP2/6-31G(d,p) level of theory while energies of the stationary points on potential energy surfaces were obtained from QCISD(T) calculations with larger 6-311++G(d,p) and 6-311++G(3df,2p) basis sets and corrected for zero-point energies. The temperature dependence of the rate constants of various dissociative processes has also been calculated using the conventional transition-state theory. While the decomposition of methyl azide occurs, in the singlet state, through a concerted motion of N-2 elimination with hydrogen shift, giving methanimine, the triplet methyl azide does not exist as a discrete species but falls apart, giving triplet methylnitrene plus N-2. Starting from singlet methanimine, 1,1-H-2 elimination giving HNC is found to be favored over 1,2-H-2 elimination giving HCN, a 1,2-H shift yielding aminocarbene, and N-H bond cleavage producing the H2CN radical. The hot HNC molecule is expected to rearrange rapidly to HCN. From singlet aminocarbene (HCNH2), 1,2-H-2 loss giving HNC is also a less energy-demanding step than the 1,2-H-2 loss, generating HCN. Overall, it appears that, in the lowest singlet state, HCN is not directly formed upon fragmentation of methanimine but rather from rearrangement of HNC which is the primary product. In the triplet state, the HCN formation from either methylnitrene or methanimine passes through successive losses of H atoms.