The relevant low-lying singlet and triplet potential energy surfaces in the photolysis of nitromethane have been studied by using the multistate extension of the multiconfigurational second-order perturbation theory in conjunction with large atomic natural orbital-type basis sets. The proposed mechanism for the photolytic decomposition of CH3NO2 provides a consistent and reinterpreted picture of the available experimental results. Two reaction paths are found in the photolysis of nitromethane after excitation at 193 nm: (1) Major Channel, CH3NO2(1A('))+hnu(193 nm)-->CH3NO2(2A('))-->IC CH3NO2(2A('))-->CH3(1A(1)('))+NO2(1 B-2(1))-->-hnu IC CH3(1A(1)('))+NO2(1 (2)A(1))-->193 nm hnu CH3(1A(1)('))+NO(A (2)Sigma(+))+alphaO(P-3)+betaO(D-1). (2) Minor Channel, CH3NO2(1A('))+hnu(193 nm)-->CH3NO2(2A('))-->CH3(1A(1)('))+NO2(1 (2)A(2))-->CH3(1A(1)('))+NO(X (2)Pi)+alphaO(P-3)+betaO(D-1), being alpha and beta fractional numbers. No ionic species are found in any dissociation path. Additionally, the respective low-lying Rydberg states of nitromethane and nitrogen dioxide have been studied too. (C) 2003 American Institute of Physics.