Acid/base properties and phenylphosphonic acid (H(2)L) complexation at the water-suspended-and-aged gamma-Al2O3/water interface have been studied in 0.1 M Na(CI) medium at 25.0 degrees C in the range 5.0 < -log[H+] < 9.5. Equilibrium measurements were performed as a series of potentiometric titrations supplemented with spectrophotometric phosphorous analyses, In the evaluation of the experimental data, the contribution from electrostatic forces was accounted for by using the constant-capacitance model. The adsorption of phenylphosphonic acid onto the studied phase was found to be strong at -log[H+] < 7. Desorption was observed when -log[H+] was increased toward 9.5. The specific capacitance was evaluated from data collected in the absence of phenylphosphonic acid. The model describing the acid/base reactions and phenylphosphonic acid binding onto the studied alumina surface, (= AlOH), is given by the following equilibria, intrinsic constants, specific capacitance, and site density : = AlOH + H+ = = AlOH log beta(110(int)) = 7.51 +/- 0.09 = AlOH = = AlO- + H+ log beta(-110(int)) = - 8.87 +/- 0.11 = AlOH + H(2)L = = AlLH + H2O log beta(011(int)) = 9.58 +/- 0.18 = AlOH + H(2)L = = AlL(-) + H+ + H2O log beta(-111(int)) = 2.94 +/- 0.08 = AlOH + H(2)L = = AlLOH(2-) + 2H(+) log beta(-211(int)) = - 5.30 +/- 0.13 C = 1.40 F m(-2) Site density = 1.03 sites/nm(2) In a series of modeling calculations, the complexation features of phenylphosphonic acid are compared to those of orthophosphoric acid.