Acid/base properties and phenylphosphonic acid (H(2)L) complexation at the boehmite (gamma-AlOOH)/water interface have been studied by means of potentiometric titrations and batch experiments. All measurements have been performed in 0.100 M Na(CI) ionic medium at 25 degrees C. The experimental data have been evaluated on the basis of the surface complexation concept with the use of the constant capacitance model to account for electrostatic contributions. It was found that the boehmite surface possesses 65% higher proton exchange capacity per surface area unit than previously recorded for an aged gamma-Al2O3 surface. This high surface concentration (0.51 mol/kg gamma-AlOOH; site density = 1.71 sites/nm(2)) was reflected in the acid/base model as a large contribution from electrostatic forces, and accordingly as a low specific capacitance value. The model describing the acid/base reactions at the boehmite surface is given by the following reactions, intrinsic constants, and specific capacitance : =AlOH + H+ reversible arrow =AlOH2+; log beta((int)) = 7.47 +/- 0.06 = AlOH reversible arrow =AlO- + H+; log beta((int)) = -9.81 +/- 0.18 C = 0.92 F/m(2). The maximum adsorption of phenylphosphonic acid was observed at -log [H+] < 7 and it was found to equal 50% of the proton exchange capacity. Within the approximate range of 7.7 < -log[H+] < 9.5, a strong decrease in adsorption affinity occurs. Data for the three-component system H+-gamma-AlOOH surface-phenylphosphonic acid can be explained with a model assuming a formation of two 1:2 (H(2)L/=AlOH) complexes according to the reactions [GRAPHICS]. No attempts were made to further adjust the acid/base model during the evaluation of the three-component model. The acid/base and phenylphosphonic acid complexation features of the boehmite surface have been compared to those of the aged gamma-Al2O3.