The formation of a metastable carbene-diazirine ylide (CDY), characterized by an UV absorption spectrum in the range of 270-290 nm and yielding azine by rearrangement, is shown to be a general process in the photolysis of diazirines. However, the yield of formation and the lifetime of CDY greatly depend on the system considered. The decreasing value of the rate constant of the reaction, carbene + diazirine --> CDY, from similar to 10(9) M-1 s(-1) for singlet dialkylcarbenes, Ad: (adamantylidene) and BCN: (bicyclo[3.3.1]non-9-ylidene), to similar to 10(8) M-1 s(-1) for benzylchlorocarbene and to similar to 10(6) M-1 s(-1) for cyclopropyl and phenylchlorocarbenes, reflects the decreasing reactivity of these carbenes. The lifetime of these ylides is determined by the value of the activation energy barrier for their rearrangement to azine, E-a approximate to 15.5 kcal/mol for dialkylcarbenes, approximate to 11.5 kcal/mol fur alkylchlorocarbenes, and <10 kcal/mol for phenylchlorocarbene. This decrease of E-a is related to the strong stabilization of the azine when proceeding from CR2=N-N=CR2 to Ph-CCl=N-N=CCl-Ph. Another mechanism for the formation of azine, by a second-order reaction of the diazo isomer of the diazirine, is clearly identified in the case of photolysis of the BCN(N-2) diazirine.