Surface complexation of orthophosphate ions at the water-suspended-and-aged gamma-Al2O3/water interface has been studied by means of a series of batch experiments in 0.1 M Na(Cl) medium at 25.0 degrees C in the range 4.8 < -log [H+] < 9.6. The ratio between phosphate concentration and concentration of surface active groups (=AlOH) was varied between 0.15 and 1.50. The suspensions were equilibrated for 5 h, and experimental data consisted of measured -log [H+] and nonbound phosphate analyses. The orthophosphate ions were found to bind to the surface with high affinity at -log [H+] < 7.5. In the data evaluation, contributions from electrostatic forces were accounted for by using the constant-capacitance model. The acid/base properties of the hydroxylated alumina surface (=AlOH) have been investigated earlier and are described by two intrinsic equilibrium constants, log beta(110) = 7.51 and log beta(-110) = -8.87 and with a specific capacitance of 1.40 F/m(2). The model describing the phosphate complexation to the alumina surface is given by the following equilibria : =AlOH + H2PO4- + H+ reversible arrow =AlPO4H2 + H2O (log beta(111(int)) = 11.49 +/- 0.08); =AlOH + H2PO4- reversible arrow =AlPO4H- + H2O (log beta(011(int)) = 5.14 +/- 0.07); =AlOH + H2PO4- reversible arrow =AlPO42- + H+ + H2O (log beta(-111(int)) = -1.82 +/- 0.04). The uncertainties reported correspond to 3 sigma(log beta). In the presence of excess phosphate and at extended equilibration periods, a slow continuing decrease in nonbound phosphate concentration was observed. By means of diffuse reflectance FTIR measurements, this phenomenon was shown to be caused by a slow transformation into an aluminum phosphate solid phase. The surface complexation reactions evaluated in this work should therefore be regarded as a metastable state, strictly valid only in freshly prepared suspensions. However, FTIR data collected at deficit phosphate conditions indicate that this phase transformation is hardly noticeable unless an excess of ligand was introduced to the system. This implies that the presented semiequilibrium model is likely to provide a thermodynamic description of the equilibria in the system for [H2PO4-](tot)/[=AlOH](tot) < 1.