An in situ infrared spectroscopic study of the adsorption of lysine peptides with n = 2-5 lysine units and polylysine (n = 169) on hydrous TiO2 particle films is reported. Using attenuated total reflection infrared spectroscopy, it was found that dilysine adsorption to negatively charged TiO2 from aqueous solutions pH 7.4 arose from electrostatic interactions. Increasing the number of lysine units to n = 3-5 resulted in infrared spectra which were changed, compared to solution species, upon adsorption to TiO2. This indicated that the carboxylate group was involved in the peptide/TiO2, interaction. Binding constants (K) for lysine peptide adsorption to TiO2 were calculated by applying the Langmuir isotherm model. The K values obtained supported the onset of a stronger peptide-TiO2, interaction going from dilysine to trilysine. Polylysine also adsorbed to TiO2 from aqueous pH 7.4 solutions. prolonged exposure of polylysine to TiO2 (16.5 h) resulted in changed infrared spectra, implying that the secondary structure of polylysine was affected by adsorption. In contrast,the lysine peptides and polylysine did not adsorb to the hydrophobic ZnSe surface or positively charged chromium(III) oxide-hydroxide films, under the same experimental conditions. This indicates that electrostatic interactions of the lysine peptides with the adsorbent are of main importance to adsorption.