In this paper, an atmospheric pressure plasma jet sustained in pure argon and an argon/water vapour mixture has been used to modify the surface of polypropylene (PP) films. The gas temperature of the plasma jet was found to be 625 K in an active zone between the electrodes and was found to increase in the afterglow. Based on these results, the PP films are placed as close as possible to the edge of the capillary in order to avoid thermal damage to the polymer. XPS results on the untreated and modified PP samples revealed incorporation of a significant amount of oxygen on the polymer surface, however, this oxygen inclusion is more pronounced for the argon/water vapour jet due to the higher radicals density in the jet afterglow. One can therefore conclude that adding water vapour to an argon plasma jet can be a convenient way to increase the efficiency of plasma surface modification. (C) 2011 Elsevier B.V. All rights reserved.