A kinetic method is described for the determination of iron(III), based on its catalytic effect on the oxidation of 3,5-diaminobenzoic acid dihydrochloride (DABA) with hydrogen peroxide. The reaction is followed spectrophotometrically by measuring the rate of change in absorbance versus time at 540 nm. The method is sensitive, selective, rapid and simple and allows the determination of as little as 1 ng ml–1 of iron(III) with good accuracy and precision. The proposed method was applied to the analysis of tap water after co-precipitation of Fe(OH)3 with Mg(OH)2 to overcome interference from calcium. The kinetic parameters of both the catalysed and the uncatalysed reactions are reported.