While some arylnitrenes generated photochemically in crystals of the corresponding azides afforded azo compounds selectively, p-(acetyl-N-methyl)aminophenylnitrene 1 yielded an azo compound AZ and an insertion product CH competitively in the crystalline environment. The selectivity was found to be controlled by the time and the temperature of irradiation. The azo compound AZ was produced predominantly (CH/AZ = 0.64) when UV irradiation was performed at cryogenic temperatures (77 K) for a long time (6 hours). On the other hand, the insertion product CH was produced exclusively (CH/AZ = 9.3), when 2 was irradiated at higher temperatures (around 270 K) for a short time (8 minutes).