We have found O-2-substituted diazeniumdiolates, compounds of structure R2N-N(O)=NOR ' that are under development for various possible pharmaceutical uses, to be rather photosensitive. With R = ethyl and R ' = methyl, benzyl, or 2-nitrobenzyl, the observed product distributions suggest that two primary pathways are operative. A minor pathway involves the extrusion of nitrous oxide (N2O) with simultaneous generation of R2N(.) and R 'O-., which may then form amines, aldehydes, and alcohols. The major reaction pathway is an interesting photochemical cleavage of the N double bondN bond to form a nitrosamine (R2NN double bondO) and an oxygen-substituted nitrene (R ' ON). The intermediacy of the O-nitrene was inferred from the production of abundant oxime, via rearrangement of the O-nitrene to a C-nitroso compound (R ' ON --> O double bond NR '), and subsequent tautomerization to the more stable oxime. Involvement of the O-nitrene was confirmed by trapping with 2,3dimethyl-2-butene to form the aziridine and with oxygen to generate the nitrate ester. 2-Nitro substitution on the benzyl derivative had surprisingly little effect on the reaction course. For each compound examined, minor amounts of nitric oxide (NO), presumably produced by secondary photolysis of the nitrosamine, were observed. Time-resolved infrared experiments provided additional support for the above reaction pathways and confirmed that the nitrosamine is a primary photoproduct. We have also found that the relative contributions of the reaction pathways can be altered in certain derivatives. For example, when R ' = 2,4-dinitrophenyl, the contribution of the nitrosamine/O-nitrene-forming pathway was diminished. Pharmacological implications of these results are discussed.