SummaryMössbauer specttoscopy has been used to provide information on the effect of pH on the nature of the complexes formed between iron and humic acid. At an initial suspension pH greater than 3 the iron occurs in the ferric form, although it is difficult to assess to what extent it is in combination with organic matter. On lowering the pH, iron is reduced with a considerable proportion of the ferrous iron entering solution, partly as a solvated ion and partly as complex forms. Raising the pH leads to re‐oxidation and the precipitation of a considerable proportion of the iron in an inorganic form with Mossbauer parameters similar to those of β‐FeOOH. No evidence was obtained for Fe(III) in solution or for Fe(II) in any form at pH values greater t