UV photodecomposition of azidomethyl methyl sulfide (AMMS) yields a transient S-methylthiaziridine which rapidly evolves to S-methyl-N-sulfenylmethanimine at 10 K. This species was detected by infrared matrix isolation spectroscopy. The mechanism of the photoreaction of AMMS has been investigated by a combined approach, using low-temperature matrix isolation FTIR spectroscopy in conjunction with two theoretical methods, namely, complete active space self-consistent field and multiconfigurational second-order perturbation. The key step of the reaction is governed by a S-2/S-1 conical intersection localized in the neighborhood of the singlet nitrene minimum which is formed in the first reaction step of the photolysis, that is, N-2 elimination from AMMS. Full assignment of the observed infrared spectra of AMMS has been carried out based on comparison with density functional theory and second-order perturbation Moller-Plesset methods.