We studied the adsorption of toluene and its photodecomposition by TiO2 surfaces under UV irradiation using P-25 samples subjected to two different treatments. In order to shed light on the elementary steps of the photocatalytic decomposition of toluene, we employed X-ray photoelectron spectroscopy (XPS). A higher population of the surface OH-group of TiO2 increased the adsorption reactivity and initial rate of the photodecomposition of toluene. We also identified more efficient accumulation of toluene on the surface of TiO2 with high concentrations of OH-groups via XPS.