Aroxyl radicals of fifteen structurally distinct flavonoids were generated by attack of azide radicals (N3.) on the parent compounds dissolved in aqueous solution at pH 11.5. Generation rate constants were all found to be very high (2.4-8.8 x 10(9) dm3mol-1 s-1), whereas the decay rates differed considerably, ranging from 10(5) to 10(8) dm3mol-1 s-1. In most cases the spectral characteristics of the transient aroxyl radicals relate to structural features of the parent compounds and according to spectral similarities they can be classed in three distinct groups (with only two exceptions). Although the data do not conclusively prove that the biological function of flavonoids might be the scavenging of radicals, the very high rate constants of formation and the relative stability of some of the aroxyl radicals, are in support of such a hypothesis.