Abstract In this work, films of a-Si:H films exhibiting low surface recombination velocities were deposited onto high lifetime silicon substrates and characterized. The films were made by plasma enhanced chemical vapour deposition with thicknesses ranging from 5 to 40nm. On one set of samples, the a-Si:H layers was capped by a ∼100nm layer of amorphous, hydrogenated silicon nitride (a-SiNx:H). The thermal stability of the surface passivation materials was investigated by minority carrier lifetime measurements. The structure and composition of the films were thereafter investigated both before and after annealing using neutron reflectometry (NR) and x-ray reflectometry (XRR) measurements. These measurements give highly accurate information of the physical structure of the films, including their thicknesses and layer densities, and also give an estimate of the H-concentration profiles within the layers. The results show that the degradation of lifetime observed after thermal processing is accompanied by a strong reduction in H concentration throughout the whole bulk of the a-Si:H films.