Laser flash photolysis studies (LFP) of O,S-diethylmonothiodiazomalonate (1) and S,S-diethyldithiodiazomalonate (2) in 1,1,2-trichlorotrifluoroethane (Freon-113) were consistent with the trapping of an intermediate with lambda (max) = 425 nm, attributed, in the LFP of I, to that of ylide 10, formed from the reaction of singlet O,S-carboethoxythioethoxycarbene (9) with pyridine. The deduced lifetime of both singlet O,S-dicarboethoxycarbene (9) and S,S-dicarbothioethoxycarbene (13) are essentially the same in Freon-113, but compared to dicarbomethoxycarbene 13, the lifetimes of 9 and 13 were found to be about six times shorter. The LFP studies. in addition to calculations at the B3LYP/6-31 G* level of theory, provide an explanation for the effect of sulfur perturbation that shortens the carbene lifetimes, as well as an explanation for the exclusive migration of the SEt group to ultimately form 5 upon light (300 nm) induced nitrogen extrusion and Wolff rearrangement of I in EtOH.