This article experimentally identifies the hydrogen incorporation and release processes which control the final hydrogen content of hydrogenated amorphous silicon films (a‐Si:H). We deposit films using reactive magnetron sputtering of a silicon target in an Ar and H2atmosphere. Hydrogen incorporation or loss is measured using real time infrared reflectance spectroscopy. An optical cavity substrate increases the sensitivity, allowing us to observe Si–H bonding in layers ≥5 A˚ thick via the stretching mode absorption (1800–2300 cm−1). We observe a narrow component at ∼2100 cm−1corresponding to all SiHxbonds on the physical surface; the line width allows us to distinguish this contribution from the broader bulk modes. Various combinations of growth flux (isotope labeling, hydrogen partial pressure between 0.1 and 2.0 mTorr) and substrate material (on SiO2,a‐Si, ora‐Si:D) at substrate temperatures between 120 and 350 °C are used to distinguish various mechanisms. From the deposition ofa‐Si:H films on SiO2, we quantify the H surface coverage at the end of the nucleation stage (10 A˚ of growth) to be 1.2±0.3×1015cm−2, essentially independent of growth conditions. From the evolution of Si–H bonding during the initial growth (≤25 A˚), we infer a reduction of the surface area (smoothening) during nucleation and coalescence. Duringa‐Si:H growth on unhydrogenateda‐Si, we observe H implantation up to a depth of 40 A˚, and derive the total flux of arriving H as a function of hydrogen partial pressure. For the exposure ofa‐Si:H to atomic deuterium and ofa‐Si:D to atomic H, we observe a loss/gain of surface H due to abstraction or exchange reactions. Whena‐Si:D ora‐Si films are deposited ona‐Si:H films, we observe H loss from the bulk of the film due to fast particle bombardment at the growing surface. Based on the experimental evidence, we discuss the dependence of various H incorporation and release processes on the incident H flux, substrate temperature, and deposition rate. ©1995 American Institute of Physics.