The mechanism of photochemical rearrangement of 3-methyl-2-(1-naphthyl)-2H-azirine (1a) into the ketene imine 2a in low-temperature matrices was investigated by matrix-isolation infrared spectroscopy. On the basis of the effects of media and the methyl-substituent, as well as the effects of sensitizer and light intensity, we propose that the rearrangement proceeds by the mechanism involving the vibrationally hot biradical 5a generated by the C-N bond cleavage of azirine in its triplet excited state. (C) 2002 Elsevier Science B.V. All rights reserved.