The kinetics of osmium(VIII) catalyzed oxidation of sulfanilic acid (p-aminobenzenesulfonic acid) by hexacyanoferrate(III) (HCF(III)) in alkaline medium was studied spectrophotometrically. The reaction showed first order kinetics in hexacyanoferrate(III) and osmium concentrations and an order of less than unity in sulfanilic acid (SAA) and alkali concentrations. The rate of reaction increased with increase in alkali concentration. Increasing ionic strength and dielectric constant of the medium increased the rate of the reaction. A retarding effect was observed by one of the products i.e. hexacyanoferrate(II) (HCF(II)). A mechanism involving the formation of a complex between sulfanilic acid and osmium(VIII) species, [(OsO5(OH)](3-) (in presence of high OH-concentrations), has been proposed. The main products were identified by spectral studies. The reaction constants involved in the mechanism are evaluated.