Laser flash photolysis of pyridine N-oxide in acetonitrile yields atomic oxygen, O(P-3), in modest quantum yield. The reactivity of O(P-3) could be probed by monitoring the product of its reaction with the solvent, assigned to acetonitrile oxide (CH3CNO). For example, reactions with dioxygen, 1-octene, and cyclopentane take place with rate constants of 4.1 x 10(9), 8.7 x 10(9), and 3.8 x 10(8) M(-1) s(-1), respectively, at room temperature. Isotope effects are ca. 6 for cyclohexane, where H abstraction is responsible for the reaction, but negligible for benzene, acetonitrile, and pyridine, where addition reactions are believed to dominate. This is the first report of an extensive set of rate constants for the reactions of O(P-3) with a variety of organic and inorganic substrates in solution.