This paper reports the results of detailed experiments carried out to understand the growth of a-Si:H films deposited by thermocatalytic chemical vapor deposition (TCCVD) at different substrate temperatures (T-s). Kinetic ellipsometry data reveal that the growth kinetics drastically changes as T-s is increased from 110 to 450 degreesC. Although nucleation occurs at all T-s, a major difference is observed in the coalescence behavior. At the lowest T-s, no trace of coalescence is reflected in the data. In the intermediate T-s region, incomplete or partial coalescence introduces the lobe/cusp pattern in the ((epsilon (1)),(epsilon (2))) curve, while above 350 degreesC, complete coalescence leads to the presence of a loop in the ((epsilon (1)), (epsilon (2))) data. We attribute this change in the ((epsilon (1)),(epsilon (2))) trajectory to a systematic variation in the surface reactivity and hydrogen coverage with increasing T-s.