Arsenate coprecipitated with iron at different pH values (pH 4.0, 7.0 or 10.0) affected the nature mineralogy, chemical composition and reactivity of Fe precipitates. In the absence of arsenate goethite (at pH 4.0 or 7.0) and goethite-hematite (pH 10.0) formed, whereas in the presence of arsenate (As/Fe molar ratio (R) of 0.01 and 0.1) the formation of ferrihydrite and hematite was promoted. At R = 0.01 (at pH 4.0 and 7.0) and 0.1 ferrihydrite formed and did not undergo any conversion to more crystalline goethite or hematite after prolonged aging (up to 7 months at 50°C). In the precipitates obtained at pH 4.0 or 7.0 arsenate was almost completely removed from the mother solutions. Its presence in the network or on the external surfaces of the precipitates strongly affected the surface properties (surface area, PZC) of the iron oxides. Powder XRD analysis of Fe-As coprecipitates formed at R = 0.1) showed an increasing degree of structural order with increasing pH. Larger amounts of phosphate were adsorbed at pH 6.0 on short-range ordered samples (ferrihydrite) formed at R = 0.01 than at R = 0.1, probably because in the latter precipitates arsenate prevented phosphate fixation on many sorption sites. Very low amounts of arsenate present in the precipitates were desorbed by phosphate (< 10 %). Larger amounts of arsenate were replaced by phosphate when arsenate was added to a preformed ferrihydrite. The desorption of arsenate by phosphate showed biphasic reaction processes: a fast reaction followed by a slow reaction.