Photocatalytic treatment of air, using TiO2 as the catalyst, is developing more and more because of increasing air pollution which is becoming a serious problem that needs to be addressed immediately. Furthermore Volatile Methyl Siloxanes (VMS) are becoming more and more abundant indoors and in ambient air owing to their increased use in different domain such as cosmetic and personnel care products. Once VMS are present in air, rapid catalyst deactivation can be observed and this reduces the efficiency of the photocatalytic system. In this study octamethylcyclotetrasiloxane D4 was chosen as the VMS model, and its photocatalytic oxidation was investigated. Experiments were performed at various D4 concentrations in synthetic air under 40% relative humidity and D4 conversion was measured using TD-GC-MS analysis, while the TiO2 surface was analyzed by X-ray photoelectron spectroscopy (XPS). Under these conditions at 250 ppbv of D4, the photocatalytic activity as measured by D4 conversion decreased from 98% to 30% after 3 days of irradiation, thus revealing the deactivation of TiO2, and total deactivation was observed after three days of irradiation at 500 ppbv of 04. The photocatalysis of toluene was studied under the same conditions over a period of three days and a conversion close to 100% was found without any deactivation, hence leading to the confirmation that only D4 contributes to the deactivation of TiO2. Additional experiments on a binary mixture of D4 and toluene allowed us to gain a better understanding of the impact of VMS on photocatalytic systems. For the photocatalytic oxidation of D4, three photoproducts were identified, viz. heptamethylhydroxycyclotetrasiloxane, heptamethyl(hydroxymethyl)cyclotetrasiloxane and hexamethylcyclotrisiloxane. XPS analyses of the media show an increase in Si/Ti ratio and the appearance of a SiOH component, which allows us to better understand the nature of the deposit responsible for the deactivation of TiO2. (C) 2014 Elsevier B.V. All rights reserved.