Laser flash photolysis of pyridine N-oxide in acetonitrile yields atomic oxygen, O (3P), in modest quantum yield. The reactivity of O (3P) 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 lo9, 8.7 x lo9, and 3.8 x lo8 M-'sl, respectively, at room temperature. Isotope effects are cu. ...