The kinetics of oxidation of ruthenium(III)-catalysed oxidation of 4-hydroxycoumarin (HDC) by diperiodatonickelate(IV) (DPN) in aqueous alkaline medium at a constant ionic strength of 1.0 mol dm(-3) was studied spectrophotometrically. The reaction between DPN and 4-hydroxycoumarin in alkaline medium exhibits 3:1 stoichiometry (DPN:HDC). The reaction is of first order in [DPN] and less than unit order in [HDC] and [alkali]. Periodate has no effect on the rate of reaction. The reaction rate increases with increase in ionic strength and decrease in dielectric constant of the medium. The main products were identified by spot test and spectral studies. A mechanism involving the DPN as the reactive species of the oxidant and a complex formation between 4-hydroxycoumarin and ruthenium(III) species has been proposed. The reaction constants involved in the different steps of mechanism are calculated. The activation parameters with respect to slow step of the mechanism are computed and discussed and thermodynamic quantities are also calculated. (c) 2005 Elsevier B.V. All rights reserved.