The first in situ internal reflection infrared spectroscopic study of lysine adsorption from an aqueous solution to TiO2 films is reported. The infrared spectrum of lysine adsorbed to TiO2 from an aqueous solution at an intermediate pH of similar to 5-7 strongly resembles that of solution lysine, indicating that lysine is electrostatically adsorbed. At this pH, favorable electrostatic interactions occur between the lysine cation and the negatively charged TiO2 film (point of zero charge at pH similar to 5). A Langmuir isotherm analysis was used to obtain an equilibrium constant for lysine adsorption at pH 7.4 of 3 x 10(3) M-1, consistent with an electrostatically adsorbed surface species. Increasing the crystallinity of the TiO2 film did not affect lysine adsorption. The surface titration by internal reflection spectroscopy (STIRS) technique hds been used to investigate the effect of pH change on lysine adsorption to TiO2. Maximum lysine adsorption to TiO2 occurs near the lysine isoelectric point of 9.8.