A polypropylene (PP) modified by ethylene-propylene rubber (EPR) was treated in vacuo by alpha particles (with an energy of 400 keV) with fluences that varied from 10(11) to 10(16) He+ cm(-2). Samples were investigated with attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, contact-angle measurements, differential scanning calorimetry, swelling, nanoindentation, and friction techniques. After treatment, new chemical groups (carbonyl and vinyl bonds) appeared at the PP/EPR surface. These chemical modifications led to changes in the surface free energy of PP/EPR. Irradiation also modified the PP/EPR structure because crosslinking occurred after treatment for fluences above or equal to 5 x 10(14) He+ cm(-2). Moreover, the PP melting temperature and enthalpy were greatly decreased for fluences above or equal to 10(14) He+ cm(-2). The surface mechanical properties were also changed after treatment. Indeed, the friction coefficient decreased, whereas the hardness and Young's modulus drastically increased, after irradiation at higher fluences. (C) 2003 Wiley Periodicals, Inc.