Mechanistic chemists have long appreciated that short lived, high energy species such as nitrenes, carbenes, and excited triplet states play a crucial role in the photochemistry of azides, diazo compounds, diazirines and various p electronic systems. The photochemistry of phenyl azide is a sensitive function of temperature. This is further demonstrated by monitoring the products formed upon photolysis of phenyl azide in the presence of diethylamine. Photolysis of the matrix isolated diazomethane derivative leads to fragmentation to form a carbene and molecular nitrogen. The low temperature photochemistry of 1 and 2-naphthyl azides differs from that observed with phenyl azide. Photolysis of triplet 1-naphthyl nitrene in an organic glass must lead, at least in part, to hydrogen atom abstraction from the matrix. Laser flash photolysis of mesitylene in a hydrocarbon solution at ambient temperature produces the triplet excited state of mesitylene whose transient absorption spectrum can be obtained in the usual manner.