The surface reactions of CH@sub x@ and SiH@sub x@ radicals as the dominant growth precursor during plasma deposition of amorphous hydrogenated carbon and silicon films are investigated by exposing a-C:H and a-Si:H film surfaces to low temperature plasma discharges or to quantified radical beams. The surface reactions are monitored in real time by using in-situ ellipsometry and in-situ infrared spectroscopy. The measurement of the surface reaction probability of various hydrocarbon radicals indicates that the reactivity of larger C@sub x>1@H@sub y@ radicals at the a-C:H surface is much higher than that of CH@sub y@ radicals. This has several consequences for the understanding of a-C:H film growth, which will be described in detail. The deposition of a-Si:H from silane discharges is assumed to be similar to the growth of a-C:H films since in both cases the dominant growth precursor is CH@sub 3@ or SiH@sub 3@, respectively. However, the dominant interaction mechanism of silyl radicals with a-Si:H surfaces, as identified by isotope labeling experiments, is very different to that of CH@sub 3@ radicals on a-C:H surfaces. Whereas methyl radicals can only adsorb at open bonds at the a-C:H film surface, silyl radicals are able to insert into strained bonds at the a-Si:H surface. A comparison of microscopic growth processes during a-C:H growth and a-Si:H growth will summarize this presentation.