Volume changes due to SO42-, SeO42-, and H2PO4- adsorption on amorphous iron(III) hydroxide were determined by dilatometry in a mixture comprising an aqueous solution of Na2SO4, Na2SeO4, or NaH2PO4 and amorphous iron(III) hydroxide suspended in 0.1 mol dm(-3) NaClO4 at an initial pH ranged from 4.50 to 6.50. System volumes increased during SO42-, SeO42-, and H2PO4- adsorption on amorphous iron(III) hydroxide, suggesting that some hydrated water around aqueous SO42-, SeO42-, or H2PO4- ions and amorphous iron(III) hydroxide were released to the bulk. The volume changes due to SO42-, SeO42-, and H2PO4- adsorption at initial pH 4.50 were + 18, +18, and +14 cm(3) mol(-1), respectively. Phosphate, which adsorbed as an inner-sphere complex, released most of its hydration water to the bulk. The volume changes due to SO42- and SeO42- adsorption indicated that they were dehydrated at the water/amorphous iron(III) hydroxide interface. The smaller changes in volume for H2PO4-compared to SO42- and SeO42- may be explained by differences of charges and adsorption mechanisms of these oxoanions.