Injection-moulded plates of ten polypropylene (PP) and thermoplastic polyolefin (TPO) materials with varying material composition (different type of rubber, varying degree of ethylene etc.) were characterized before and after oxygen plasma treatments. Untreated materials were studied by means of differential scanning calorimetry (d.s.c.), size exclusion chromatography (s.e.c.), Fourier-transform infrared spectroscopy (FTi.r.), attenuated total reflectance (ATR) and transmission measurements, and the effect of plasma treatment conditions was followed by X-ray photoelectron spectroscopy (X.p.s.) and contact angle measurements. S.e.c. analysis revealed only minor variations among the materials, while the d.s.c. and FTi.r. experiments confirmed that the differences were to be expected as a result of the variation in material composition. The FTi.r.-ATR results showed that all samples had a gradient in material composition. The materials were generally more rich in PP in the topmost similar to 200 Mil than in the first similar to 800 nm, and a lesser extent of ethylene modification and/or rubber was observed in the topmost similar to 200 nm. It was also shown that the degree of surface crystallinity was normally greater at similar to 800 nm than at similar to 200 nn, and that a higher mould temperature led to a higher degree of surface crystallinity. The water contact angles and the atomic composition showed that the materials were more oxidized after plasma treatment at high power-to-gas pressure (P/G) ratios than at low ratios. Moreover, the dependence on material composition was weak for samples that were plasma-treated at low P/G ratios whereas the materials that were least ethylene-modified were less oxidized than the others at high P/G ratios.