Nanocrystalline hydrogenated silicon films were grown by radiofrequency magnetron sputtering in an mixed Argon/Hydrogen (30%/70%) plasma. For some series Helium or Nitrogen was added to the gas mixture. The films were grown at 250 °C for total different pressures varying from 5 to 15 Pa. The crystalline fraction increases with pressure from 85 to 93% but at the same time the porosity of the films increases from 11 to 19%. This relatively high porosity leads to an important post-growth contamination of oxygen as revealed by IR absorption spectroscopy and nuclear reaction analysis (NRA) and can explain the low stress observed in these films. The Electron spin resonance performed at 35 GHz indicates the presence of oxygen in the surroundings of the paramagnetic defects. The spin density is of the order of some 1017 cm−3, independent of the growth pressure and in agreement with photothermal deflection spectroscopy results. From the IR spectroscopy and NRA one can deduce that most of the oxygen in the films is bound to silicon. Helium dilution of the plasma at 5 Pa tends to reduce the porosity and the oxygen contamination in the film but above 30% He dilution, the crystalline fraction decreases. When nitrogen is introduced into the plasma during growth the films become amorphous, even for very small partial pressures of N2.