Ultra-high molecular-weight polyethylene (UHMWPE) and linear polyethylene (LPE) were treated by low keV H 2 +, He + and N 2 + ion beams up to 2 × 10 17 ions cm −2, and the chemical changes induced in the modified surface layer were studied by FT-IR and XPS. The concentration of vinyl groups decreased, while that of the trans-vinylene, trans-trans diene, methyl and carbonyl groups increased. The degree of polymer crystallinity was reduced. Changes in the valence band, in the C KVV Auger peak and in the plasmon loss splitting of the C1s peak indicated the occurrence of graphitisation. On treatment by N 2 + ions – in addition to the above changes – nitrogen incorporation took place. The N-content of the surface layer reached a saturation value of ∼11 at% at ∼10 17 ions cm −2. The N1s peak was decomposed to three components at 398.3, 400 and ∼402.8 eV, which were assigned to nitrogen in C–N, CN (imine), and CN with delocalised nonbonding electron pair on N, respectively.