Sum frequency generation (SFG) vibrational spectroscopy is used to investigate the surface adsorption of three probe molecules-acetic acid, methanol, and water-on a film composed of nanoscale anatase TiO2 particles. On the TiO2 surface, only one adsorption mode, chemisorption, is observed for acetic acid. This is evidenced by one sharp SFG peak in the C-H region, which is stable with time and robust both to evacuation and to the addition of water. A Langmuir constant of (9.21 +/- 0.71) x 10(3) is determined from the adsorption isotherm. In the case of methanol adsorption, however, there are two adsorption modes, molecular physisorption and dissociative chemisorption. The corresponding SFG signals are stable with time but diminished with addition of water. Changes in the SFG features for methanol and for the methoxy species with addition of water and subsequent evacuation provide the first experimental proof of reversible hydroxylation and dehydroxylation at the TiO2 surface. For water adsorption, only one mode, physisorption, is observed on the hydroxylated TiO2 surface. The water adlayer is mobile, as is evidenced by variation of the water H-bonded SFG signal with time. Competitive adsorption among the three molecular probes is clearly resolved by in situ SFG measurements. The adsorption strength follows the order acetic acid (strongest), methanol, water (weakest). The adsorption order as well as the difference in response of methanol versus acetic acid adsorption to addition of water has direct implications for understanding TiO2 photocatalysis as well as the surface modifications involved in TiO2 photoelectrochemical solar cells and processes in TiO2 nanomaterial synthesis and assembly.