The mechanism of photodecomposition of trans-azomethane (CH3-N=N-CH3-->2CH(3) .+N-2 has been investigated with high level ab initio molecular orbital calculations. Potential surfaces of the low-lying electronic states were explored by state-average complete active space self-consistent-held (sa-CASSCF) and multireference configuration interaction with single and double excitation (MRCISD) methods. The calculated vertical excitation energies for S-0-->S-1 and S-0-->T-1 transitions are in good agreement with experiments. The lowest crossing point between the S-0 and S-1 surfaces, around which excited molecules would make efficient internal conversion to the ground state, is found to be asymmetrical with a CNNC dihedral angle of 92.8 degrees and two CNN angles of 132.0 degrees and 115.6 degrees, respectively. Transition structures for both simultaneous and sequential C-N bond cleavages on the S-0 surface were found. Though the activation energy of sequential C-N bond cleavage is about 7 kcal/mol higher than that of the simultaneous C-N bond cleavage, the Gibbs free energy of activation is lower above 0 degrees C, indicating that thermal decomposition of trans-azomethane is sequential. Photodissociation is expected to take place sequentially as well. In the sequential mechanism, dissociation of the first C-N bond on the S-0 surface takes place endoergically without reverse barrier resulting in CH3N2 intermediate, which should decompose almost immediately over a barrier of less than 1 kcal/mol. Thus, the photodissociation reaction is highly asynchronous but is nearly concerted. This mechanism can explain two seemingly contradictory photodissociation experiments that two methyl radicals have very different translational as well as internal energies and that the velocity vectors of the three fragments are strongly correlated.