Various partial oxidation products were identified on the surface of TiO2 and an 8% SiO2-TiO2 binary catalyst used for the photocatalytic oxidation of gas-phase toluene. Using in situ FTIR spectroscopy, benzaldehyde and benzoic acid were identified on the surface of the deactivated photocatalysts. Additional GC/MS analysis of methanol-extracted surface species confirmed the presence of benzaldehyde and benzoic acid and detected small concentrations of benzyl alcohol. Apparently, benzaldehyde is the main partial oxidation product that is further oxidized to benzoic acid. Benzoic acid is strongly adsorbed on the surface of the catalyst. There seems to be a correlation between the accumulation of benzoic acid on the surface and catalyst deactivation. The presence of gas-phase water in the reactive mixture seems to retard the formation of benzoic acid.The SiO2-TiO2 photocatalyst is more active and appears to deactivate slower than TiO2. This binary oxide is photocatalytically active even in the absence of gas-phase oxygen. It also seems to have a higher toluene adsorption capacity than TiO2. The acidity of the different oxides was examined using FTIR spectroscopy of adsorbed pyridine. The results indicate that no pure metal oxide displays Bronsted acidity but when SiO2 is cofumed with TiO2, Bronsted acidity of intermediate strength is generated. The generation of new surface sites may be responsible for the increased activity. The mechanism of this promotion effect is not clearly understood and further studies are required to elucidate it.