Complexation of o-phthalate (1,2-benzenedicarboxylate) at the water-boehmite (gamma-AlOOH) interface has been studied in 0.1 N NaCl media at 298.2 K within the range 4.0 < pH < 10.0. Equilibrium measurements were performed as potentiometric titrations and adsorption measurements. Attenuated total reflectance IR spectra were recorded to obtain structural information of the phthalate surface complexes. Additional adsorption and IR spectroscopic measurements were performed as a function of ionic strength. Infrared data strongly indicated the formation of two complexes at the surface, one outer-sphere and one inner-sphere complex with a chelating bidentate structure. The relative concentrations of these complexes were shown to vary with ionic strength and pH. The potentiometric and adsorption data were modeled using the extended constant capacitance approach to describe the electrostatics at the water-boehmite interface. Furthermore, constraints from the infrared spectroscopic results were used in the model. The surface complexation model which provided the best fit to all experimental data included the following equilibria: =AlOH + HL- <-> =AlOH2+L2- (outer sphere); =Al(OH)(2) + H+ + HL- --> =AlL + 2H(2)O (inner sphere).