The kinetics were studied of the following reactions: OH + CO -> CO2 + H (at 220-900 K), OH + CH4 -> H2O + CH3 (at 300-900 K), OH + NH3 -> H2O + NH2 (at 228-472 K), and OH + HNO3 -> H2O + NO3 (at 240-406 K). The studies are related to reaction in the atm. The OH radicals were generated by flash photolysis of mixts. contg. H2O vapor. The subsequent decay of OH radicals by the above 4 reactions was followed by time resolved resonance absorption in the A2S+ -> x2P system of OH.