I n situ single-wavelength ellipsometry measurements have been used to monitor the growth of thin films of hydrogenated amorphous silicon (a-Si:H) deposited on c-Si substrates using an rf glow discharge. Detailed results are presented for two depositions: one from pure SiH4 and the other from SiH4 diluted in Ar (1:10). Other parameters were fixed to obtain material with optimal photoelectronic properties. At the outset of deposition, for even the highest quality material, a nucleation process is observable in the first 65(±10) Å which causes significant deviations of the in situ ellipsometry data from that calculated using thickness independent optical constants. The deviations are modeled in terms of changes with film thickness in bulk density deficits and surface roughness layers. Spectroscopic ellipsometry analysis of films of different thicknesses prepared under identical conditions are presented to support our models. The oxidation behavior of a-Si:H in an O2 plasma, also presented, provides information on the structure of the surface roughness on the films.