Abstract A flow injection-photometric method has been developed for the determination of iron(II+III). The method is based on the catalytic effect of iron(III) on the hydrogen peroxide oxidation of 3,3′,5,5′-tetramethylbenzidine to form a blue compound ( λ max=650 nm). In this catalyzed reaction, 1,10-phenanthroline acted as an effective activator. Iron(II) is also determined, being oxidized by hydrogen peroxide. Calibration graphs for iron(II) and iron(III) obtained under the optimized conditions were identical with each other and linear in the range 0.2–200 ng ml −1 with a detection limit of 0.05 ng ml −1 iron. The reproducibility was satisfactory with a relative S.D. of 1.0% for ten determinations of 5 ng ml −1 iron(III). The proposed method was successfully applied to the determination of iron in river and lake water samples and can be determined free iron species.