Reactions of ground state oxygen and hydrogen atoms produced by an rf electric discharge were studied in a flow system at pressures of about 1 mm Hg. Hydroxyl radicals and oxygen atoms are generated in the H +; NO2 reaction and measurements of the rate of removal of oxygen atoms in this system led to a value for the rate constant k3 of the rapid reaction O +; OH → H +; O2 (3) of 3 ± 1 × 1013 cm3mole–1sec–1 at 293°K and 265°K. The reaction of H with O2 in the presence of an argon carrier was found to have over-all third order kinetics at 293°K, H +; O2 +; Ar → HO2 +; Ar (7) and under these conditions k7 was found to be 1.2 ± 0.3 × 1016 cm6mole–2sec–1. This rate constant is similar in magnitude to that of the analogous combination reaction, H +; NO +; Ar → HNO +; Ar. The rate of reaction of oxygen atoms (in the absence of O2) with molecular hydrogen was measured between 409° and 733°K. Assuming the slow primary step (–4) O +; H2 → OH +; H (–4) to be followed by the rapid reaction O +; OH → O2 +; H a value of k–4 = 1.2 × 1013 exp [(–9200 ± 600)/RT] cm3mole–1sec–1 was determined in this temperature range.