The oxygen to silicon ratio of several SiO x H y thin films deposited by the electron cyclotron resonance plasma method was studied by several methods (heavy ion elastic recoil detection analysis, energy dispersive X-ray spectroscopy, Auger spectroscopy and infrared spectroscopy). Among these methods, other groups found that x scales linearly with the wavenumber of the Si–O–Si stretching vibration (v st ) by the relation x=0.020v st −19.3. This equation has been used by many different groups to determine x of SiO x thin films, but we have found that in our ECR deposited films the above mentioned formula gives accurate results for x values higher than 1.5, but for Si richer films the formula overestimates the x value, with values well outside the 20% accuracy range. A possible explanation of this discrepancy may be the bonded hydrogen of the films: in the plasma deposited samples used in this study the hydrogen content was high, above 20 at.% for silicon-rich samples. As a consequence, the Si–O–Si groups were immersed in a more electronegative matrix than in the usual case (SiO x with a low hydrogen concentration) and thus the variation of the position of the stretching peak was less pronounced.