A comprehensive speciation scheme was proposed for the speciation of iron in river water (0.45 µm filtrate). The speciation was based on the catalytic determination of reactive iron as Fe3+ and FeIIILi(3 −
in)+, where i
= 1 or 2 for a unidentate and i
= 1 for a bidentate ligand, Ln−. The scheme incudes size fractionation and acid decomposition of the sample and determination of the coexisting species. The proposed scheme was applied to natural and artificial samples and acidified preparations of these. Resulting fractionation patterns of reactive and unreactive iron and of coexisting humic acid were useful for the characterization of iron species in the natural samples. The reactive iron in the molecular weight (Mr)
≥ 104 fraction was associated with humic iron aggregates
and estimated as iron complexed with them. The unreactive iron in this fraction dissociated in 0.1 M HCl and was estimated as iron in the aggregates of iron(III) hydroxide (s) and humic iron. From the fractionation results of the artificial sample, the reactive and unreactive iron in the Mr < 103 fraction may be formed by complexing simple anions like F− and C2O42− at their naturally-occurring concentration levels. The fractionation results also suggested that humic acid and Si contributed to the formation of reactive and unreactive iron in the Mr
≥ 104 fraction.