Poly(ethylene terephthalate) (PETP) was treated by plasma immersion ion implantation (PIII or PI3) in nitrogen. The surface changes were characterised by XPS and water contact-angle measurements. Sliding tribological properties of untreated and nitrogen PIII-treated PETP against conventional low carbon structural steel S235 were studied under dry and water-lubricated conditions by a pin-on-disc tribometer. XPS results suggested the evolution of surface composition and bonding towards those of amorphous hydrogenated carbon-nitride. Water contact-angle decreased implying increased surface wettability. At a very low Pv factor (0.0075 MPa m s (1)) for the nitrogen PIII-treated PETP the dry friction coefficient was smaller than, while the lubricated friction coefficient was similar to, the corresponding value of the untreated variant. At higher Pv factors (near 0.1 MPa m s (1)), however, both the dry and lubricated friction coefficients were higher for the treated sample than for the untreated variant, suggesting an increased adhesion component of friction for the nitrogen PIII-treated PETP in this region. (C) 2009 Elsevier B. V. All rights reserved.