The early stage of the growth of plasma deposited amorphous silicon (a‐Si:H) on glass substrates is investigated byinsituinfrared phase modulated ellipsometry in the silicon–hydrogen stretching mode region. Analysis of the spectra provides unprecedented sensitivity and quantitative information on the film evolution. In particular SiH, SiH2, and SiH3bonds are identified in 5–20 A˚ thick samples. The bond densities of SiH and SiH2in thin films are estimated. After the interaction with the substrate,a‐Si:H films grow beneath a hydrogen rich overlayer containing SiH2and SiH3bonds. At 250 °C the thickness of this overlayer is compatible with one monolayer. The hydrogen‐passivated surface ofa‐Si:H is then weakly reactive.