Decay curves have been measured for reaction with oxygen of radicals derived by pulse radiolysis from a series of bipyridylium salts. The shapes of the curves were consistent with decay due to reaction of the bipyridyl radicals with both oxygen and superoxide ion, and rate constants k1 and k2 for these reactions were estimated. For compounds with redox potentials above –0.2 V, the reverse reaction, between superoxide ion and bipyridylium salt, was included in the kinetic scheme. k1 values were mostly between 4 and 9 × 108 dm3 mol–1 s–1 and k2 values were all higher. The correlation of rate constants with difference between redox potentials of donor and acceptor is discussed.