The formation of a graft polymer is observed when polypropylene as a film reacts with maleic anhydride in the vapour phase in the presence of peroxide. The activity of the employed peroxides decreases in the order: dicumylperoxide > bis(tert, butylperoxyisopropyl)benzene > benzoylperoxide, The grafting reaction is not limited to the surface, but also takes place inside the film. Besides this reaction takes place on the film surface the formation of groups which give in ATR spectra a diffuse absortion band in the range between 1664 and 1500 cm(-1). The modified film is characterised by a wetting tension of 35 dynes/cm and exhibits the anhydride group characteristic properties.