Photoirradiation of aqueous suspensions (pH 3 and 11) of TiO2 on which octaphenylcyclotetrasiloxane (OPCTS) and poly(methylphenylsiloxane) (PMPS) have been deposited leads to quantitative formation of silicates and carbon dioxide. The absence of appreciable amounts of water soluble aromatic compounds and the formation of large amounts of non-monomeric silicates after irradiation suggest that the mechanism of degradation involves the attack of the active species on the aromatic substituent and the simultaneous/subsequent silicone ring opening for OPCTS or chain hydrolysis for PMPS. TiO2 photoactivity does not change significantly during and after silicone degradation, and soluble silicates do not coat the photocatalytic surface. Photodecomposition of silanized TiO2 powders in aqueous suspension was determined through the release of silicates under irradiation and by a check of the catalytic activity. After 2 h of irradiation the silanized TiO2 recovers the catalytic properties of untreated TiO2.