Plasma-deposited silicon oxynitride films have been annealed in the temperature range 800–1000°C in a D 2/N 2 gas mixture in order to study the H-D exchange reaction in the surface region of these materials and the diffusion rate of hydrogen in these materials. The hydrogen and deuterium depth profiles and the H and D bonding configurations were examined using Elastic Recoil Detection and Fourier Transform Infrared Absorption Spectroscopy. In the as-deposited materials the diffusion of hydrogen appeared to be very fast. This allowed a separate study of the hydrogen-deuterium exchange reaction. We deduced the activation energy for the reaction of nitrogen-bonded hydrogen in the material with D 2-bonded deuterium from the gas phase to be 1.5 eV. In the pre-annealed oxynitrides the diffusion rate appeared to be decreased down to values of 10 -12 to 10 -13 cm 2/s in the range 900–1000°C. Both the exchange rate and the diffusion coefficient increase with the O/(O + N) concentration ratio for O/(O + N)>0.3.