Abstract The photochemistry of the α-diazo sulfoxides 1a and 1b has been investigated in solid argon at 10 K by IR and UV/Vis spectroscopy. Both diazo compounds react to form sulfine 2a by photochemically induced hetero Wolff rearrangement of sulfinyl carbene 3, which itself cannot be detected in solid argon. Irradiation at λ> 320 nm results in decomposition of the sulfine 2a to give 2-oxa-4-thiabicyclo [4.4. 0] decane-3.5-dione (5) ...