The product distribution in the photolysis of methyl (p-nitrophenyl)diazoacetate in MeCN-MeOH (9:1) was dramatically changed by the addition of an electron-donating amine, where the formation of the products derived from the carbene was quenched completely. On the basis of the direct observation of transient species and the dependence of the products on amine concentration, the reaction scheme involving a single electron transfer to afford the carbene radical anion is proposed.