Benzylchlorocarbenes generated photolytically from diazirines in EtOH afford chlorostyrenes and acetals, whose distributions are sensitive to the reaction temperature. In the past few years several research groups have studied the chemistry of carbenes in rigid matrix at low temperature. l-4 In the case of carbenes with triplet ground states there are profound differences between the chemistry observed in fluid solution at ambient temperature and that obtaining in matrix at -196°C. For example, arylcarbene generated in alcoholic solution at ambient temperature gives O-H insertion product almost exclusively which is mostly derived from the singlet state, whereas arylcarbene generated in alcoholic matrices at -196°C resulted in a dramatic increase in C-H insertion products at the expense of the ether. These C-H insertion products are proposed to be formed via a H atom abstraction of the triplet carbene. More recently, it has been brought to our attention that the migratory aptitude and stereochemsitry in 1,2 migration of arylcarbene system were also dependent upon temperature.5 Thus, the migratory aptitude trends observed for arylcarbene (A) changed from H>Ph>>Me to Ph>H>>Me as the temperature is lowered. In contrast to these intensive studies lavished upon the matrix reactions of arylcarbene with triplet ground state, only scant information6 is available on the effect of temperature on the chemistry of carbene with singlet ground state. In this work we have examined the effect of temperature on the reaction patterns of benzyl-(Al R=H, Me (B) chlorocarbene (B), a potent singlet ground state carbene , in order to learn more about the carbene chemistry in matrices. Irradiation of 3-benzyl-3-chlorodiazirine (la)7 in cyclohexane was carried out with a 300-W high-pressure Hg lamp at 0°C until all of the diazirine was destroyed. A Corning CS-052 filter cutoff at 350 nm was used in order to avoid product isomerization. The photolysis products were exclusively E-and Z-B-chlorostyrenes (3a), which are apparently arising from