Poly(ethylene-glycol) (PEG) is a material allowing reliable reduction of protein adsorption. A comparison between the transformations induced by ion beam (IonB) and plasma modification (PlM) is presented in this work. Spin casted PEG films deposited onto Si(1 0 0) have been bombarded with an Ar and Ar-N-2 ion beams (500 V). Another series of samples was modified in parallel by Ar plasma in an electron cyclotron resonance reactor (1000-1400 W). Bulk and surface techniques were used to describe the changes induced on the PEG films. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectra (XPS) outlined relevant changes in the polymer network. From the point of view of the applications, both IonB, and PlM induced an increase in the water contact angle with respect to untreated PEG films, which is interpreted as an increase of their stability. Time of flight-secondary ion mass spectroscopy results of the processed PEG films confirmed the overall fragmentation of the polymer. The correlation with those results obtained by FTIR and XPS support that crosslinking and condensation are the main transformations induced in the films. (C) 2004 Elsevier B.V. All rights reserved.