Silicon (oxy)nitride films (SiOxNy) have been deposited onto silicon by low‐pressure chemical vapor deposition using SiH2Cl2, N2O and NH3 or ND3. Nuclear reaction analysis, elastic recoil detection, and Rutherford backscattering spectrometry have been used to determine the elemental composition of the films with emphasis on the hydrogen and deuterium content. In the as‐deposited, NH3‐grown films the bulk hydrogen concentration is about 3 at. % for an oxygen/nitrogen atomic ratio (O/N) smaller than 0.4, for O/N≫0.4 it is lower. In 900 and 1000 °C vacuum annealed films the bulk hydrogen concentration as a function of O/N goes through a maximum at O/N≊0.4. By comparing this observation with the D content in ND3‐grown films as a function of O/N, a model is deduced which explains this behavior. This model involves an oxygen induced increase of the electronegativity of the atoms to which hydrogen/deuterium is bound. Annealing at 1000 °C in a H2/N2 gas mixture of NH3‐grown films results in bulk hydrogen concentrations ranging between those measured after the 1000 °C vacuum anneal and the values for the as‐deposited state. The chlorine concentration increases with increasing oxygen content in the oxynitride films from 0.04 at. % at O/N=0 to 0.65 at. % at O/N=∞. Implications of the data and the proposed model for the electrical performance of the silicon (oxy)nitrides are briefly discussed.