The photodissociation of methyl thionitrite (CH3SNO) has been computationally studied by means of CAS-SCF and MS-CASPT2 methods. The analysis of the multiconfigurational wavefunction corroborates the assignments of the absorption spectra, the visible and UV bands assigned to S-0 , S-1(n, pi*) and S-0 -> S-2(pi, pi*) transitions correspond to 1A' -> 1A ''(n(sigma), pi*) and 1A' -> 2A'(n(pi), pi*) excitations, respectively. With respect to the photochemistry of CH3SNO, it is found that the potential energy surfaces associated with the low-lying excited states of this molecule (1(1)A '', 2(1)A', and 2(1)A '') are repulsive along the NO elimination coordinates. For this reason, population of such excited states leads to NO extrusion as the primary process, in agreement with experiments. (C) 2012 Elsevier B.V. All rights reserved.