Thin spin coated films of high-density polyethylene (HDPE) were modified by argon and oxygen radio frequency glow discharge plasma. The modified polymer surfaces were characterised by in situ X-ray photoelectron spectroscopy (XPS), contact angle measurements and time-of-flight secondary ion mass spectrometry (TOF-SIMS). In contrast to other reports of argon-plasma treated polymer substrates, we have been able to treat HDPE films with argon-plasma without oxygen incorporation, as measured with in situ XPS. However, exposure to air of argon-plasma treated surfaces leads to incorporation of oxygen (and some nitrogen) species. Properties such as amount of oxygen incorporation as a function of plasma treatment time, oxygen saturation levels, ageing and modification depth were studied. (C) 2002 Elsevier Science B.V. All rights reserved.