Ultra-high molecular-weight polyethylene (UHMWPE) was treated by fast atom beams (FAB) obtained from He, Ar, H, and N-2 With about 1 kV accelerating voltage and estimated fluence of 10(17) particles cm(-2). The modified surface layers were characterised by X-ray photoelectron spectroscopy (XPS or ESCA) and dynamic microhardness measurements. Each applied FAB treatment results in the increase of the bulk plasmon loss energy (E,) of the C Is peak. This implies the formation of graphitic-type material and/or hydrogenated amorphous carbon (or carbon nitride in case of treatment by N atoms) in the modified surface layer. FAB treatment by N atoms leads to the incorporation of approximate to 35 at.% of N, which is about three times higher than the value obtained previously after low-kilo-electron-volt N: ion-beam treatment. The related C Is peak shows that the overwhelming portion of C is bound to N, while the N Is peak reflects that N is present in at least three chemical bonding states. Angle-dependent XPS studies of the nitrogen-FAB-treated UHMWPE reveal the presence of a N-rich subsurface layer with a topmost layer containing less N. This is in agreement with the calculated depth distribution of N atoms. Each applied FAB treatment leads to significant increase in the surface microhardness.