The incorporation of hydrogen in nearly stoichiometric silicon nitride deposited by plasma enhanced chemical vapour deposition is investigated by infrared spectroscopy. During the deposition, deuterium is added to the silane, nitrogen and helium mixture. After annealing up to 1000 degrees C, some films show a very large increase of the 2200 cm(-1) absorption peak. We conclude from analysis of isotopic substitution that the free hydrogen atom does not contribute to building new Si-H bonds. This result agrees with our previous conclusion on a continuous and large change in the oscillator strength of the Si-H stretching mode.