The effects of ion irradiation on the composition, structure, compactness, and surface hardness of polyorganosiloxane films synthesized by plasma-enhanced chemical vapor deposition were investigated as a function of the ion mass and fluence. The films were obtained from a glow discharge plasma of a hexamethyldisiloxane (HMDSO)-O-2-Ar mixture, and the irradiations were carried out with 170 keV He+, Ne+, Ar+, and Kr+ ions at fluences between 1 x 10(14) and 1 x 10(16) cm(-2). To characterize the film elemental composition, two ion-beam analysis techniques were used: Rutherford backscattering spectroscopy (RBS) and forward recoil spectroscopy (FRS). The ion-beam-induced hydrogen loss from the films was significant. For the He+-irradiated samples, a H loss of about 50% with respect to the pristine or unirradiated film was observed for the highest fluence. The surface hardness measurements, performed with a nanoindenter, in films irradiated at a fluence of 1 x 10(16) cm(-1) were 8.1, 6.0, 4.7, and 1.6 GPa for He+, Ne+, Ar-,(+) and Kr+, respectively. To examine the ion-induced structural transformations in the films, infrared reflection-absorption spectroscopy (IRRAS) was employed. From analysis of the spectra of the irradiated samples several conclusions could be drawn. For example, as the ion fluence increased, (i) the densities of methyl- and Si-O-related groups changed, (ii) film disorder increased, and (iii) groups such as Si-CH2-Si, and Si-OH, which were not present in the pristine film, were formed at lower fluences but disappeared when the latter attained their highest values. Furthermore, some of the absorption peaks that appeared at low fluences and increased with increasing fluence strongly indicate formation of carbon domains in the film. Finally, differences in the ion-induced modifications produced by the different ion species were analyzed in terms of the electronic and nuclear collisions of the ions traversing the film using the well-known SRIM simulation program.172357895797