The adsorption of L-aspartic acid (AA) at the TiO2-KNO3 aqueous solution interface was studied as a function of pH and electrolyte concentration using direct adsorption, OH- desorption, electrophoresis, and XPS measurements. A multisites surface complexation model was used to describe the charging and adsorptive behavior of the particles. Adsorption and electrophoretic data show that the amino acid uptake is characteristic of adsorbing anions and almost ionic strength independent. Although XPS data indicates that AA interacts with TiO2 surface sites through the amino groups, neither XPS nor kinetic results give conclusive evidence for the adsorption mechanism (ligand exchange or hydrogen bonding). Model. calculations together with OH- desorption data, however, allow us to conclude that ligand exchange reactions, by which AA forms inner sphere surface complexes, take place. The model also predicts that only terminal (TiOH) groups react with AA. The results exemplify the usefulness of modeling when a quite complete set of experimental data cannot elucidate the type of reaction taking place between the solute and the surface.