The photolysis of HNCO vapor has been studied in the presence of ethylene, butene, ethane, propane, and neopentane at 51 °C. Both the olefins and paraffins reduced the yield of CO to about one-third of its value in pure HNCO, which is thought to correspond to the CO formed in the primary photolytic step. Yields of N2 and H2 were reduced close to zero in the presence of olefins, and it appears that NH radicals were removed by reaction with olefins. No imines, amines, nor other nitrogenous products could be detected, however, and it is suggested that these were lost by polymerization.Yields of N2 were also reduced close to zero in the presence of paraffins, and it appears that NH reacted readily with the hydrocarbons. Once again, no amines could be detected. The yield of H2, a minor product in pure HNCO, was markedly enhanced, and alkyl radical dimers appeared as products. A mechanism is suggested which involves the decomposition of vibrationally excited amines formed by insertion reactions.