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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 kl 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. ki values were mostly between 4 and 9 x lo8 dm3 mol-' s-' and kz values were all higher.The correlation of rate constants with difference between redox potentials of donor and acceptor is discussed.

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PYR1DIr:rusi AND EIPYRIDYLIUM ions readily undergo oneelectron reduction to give stable free-radicals and cation radicals respectively.' The one-electron reduction2 of 1,l'dimethyl-4,4'-bipyridylium dichloride (paraquat dichloride) is of great interest because of the correlation between ease of reduction and herbicidal activity, in a range of bipyridylium salts3 Since the oxygen-sensitive cation-radical (111) produced by one-electron reduction of paraquat (I) has an