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The original ferrozine method has been modified to sequentially determine the Fe(II)/Fe(III) speciation in small volumes of fresh and marine water samples, at the submicromolar level. Spectrophotometric analyses of the Fe(II)–ferrozine complex are performed on a single aliquot before and after a reduction step with hydroxylamine. The procedure is calibrated using Fe(III) standards stable under normal conditions of analysis. It is shown also that the presence of high concentrations of dissolved NOM (natural organic matter) do not create any significant artifacts. The method was used to measure Fe(II) and Fe(III) depth distribution in salt marsh pore waters and in a stratified marine basin.