Volume changes on H2PO4- adsorption with amorphous iron(III) hydroxide were determined in a mixture of aqueous solutions of phosphates (NaH2PO4) and suspended amorphous iron(III) hydroxide at an initial pH of 5.50 or 4.50, at 298 K. Volume changes were determined by a dilatometer after the mixing of phosphate solutions and suspended amorphous iron(III) hydroxide solutions. The increased volumes of the systems after the mixing of H2PO4- solutions and suspended amorphous iron(III) hydroxide suggested that H2PO4- adsorption was accompanied by dehydration of aqueous H2PO4- ions and suspended amorphous iron(III) hydroxide in water. The estimated volume changes per mole of H2PO4- adsorption were about +15 cm(3).