With a modified attenuated total reflection infrared absorption (ATR-IR) method, vibrational spectra and adsorption isotherms of sulfate, acetate, and oxalate on TiO2 (Degussa P-25, mostly anatase) were measured. For oxalate, the dependence of the spectral shape on the surface coverage is consistent with the formation of several surface complexes. The method uses a horizontal attenuated total reflection (HATR) element coated with a layer of highly dispersed solid material and allows quantitative in situ FTIR measurements of adsorption as a function of solution parameters (adsorbate concentration, pH, ionic strength, etc.). Sets of FTIR spectra obtained under varying conditions are analyzed using singular value decomposition (SVD) and global analysis with acid-base and various adsorption isotherm expressions. This allows for a separation of spectral contributions from dissolved and adsorbed species and of species with distinct structures and/or different adsorption equilibrium constants. The vibrational spectra of adsorbed oxalate suggest that several forms of inner spherically bound surface complexes are formed at lower pH values. This can be explained by the formation of protonated and unprotonated surface complexes and by the heterogeneity of the TiO2 used (Degussa P-25 anatase contains ca. 15-30% rutile). We compare the spectra of surface-bound oxalate species with those in aqueous solutions and discuss possible structural assignments.