The kinetics of the oxidation of hydroquinone (H 2 Q), catechol (H 2 cat) and substituted catechols with nickel(III) macrocycle [NiL 1 ] 3? (L 1 = 1,8-bis(2-hydroxyethyl)-1,3,6,8,10,13-hexaazacyclotetradecane) has been studied spectrophotometrically in aqueous perchloric acid in the presence of sulfate. Over the pH range 1-2.6, and at higher sulfate concentration 0.05 \ [SO 42-] \ 0.2 mol dm -3 , the decomposition of [Ni III L 1 ] in the oxidation of benzenediols via [Ni III-L 1 (OH)(H 2 O)] is small. With hydroquinone, the rate constant is almost independent of the hydrogen ion concentration and with catechols, the involvement of catechol anion (Hcat -) has been considered. The rate constants for cross reactions are discussed in terms of the Marcus relationship.