Non-thermal plasma as well as photocatalysis is intensively investigated for volatile organic compounds (VOCs) oxidation. Their association exhibits high performances for air pollutant removal and mineralization. Nevertheless, numerous experimental investigations are performed under dry conditions, or on very short range of water vapour amounts. This article aims at determining the influence of water vapour on the oxidation efficiency of (i) photocatalysis, (ii) non-thermal plasma, and (iii) their association. The amount of water vapour investigated in dry conditions ranges from 0 ppm to 23,000 ppm. Acetylene has been selected as a model pollutant. The presence of watervapour in the gas stream induces a strong decrease in the photocatalytic oxidation of acetylene. The mineralization process is modified. The efficiency of acetylene removal by non-thermal plasma decreases regularly with the water vapour amount. Nevertheless, the presence of water vapour improves significantly the selectivity of carbon dioxide. The same tendency is reported for plasma/photocatalysis association. Those results are interpreted in terms of oxidative species modification due to the presence of water. Moreover, the investigation of water vapour contribution into oxidative processes improves the understanding of plasma/photocatalytic association mechanisms. (C) 2008 Elsevier B.V. All rights reserved.