Electron spin resonance (ESR) methods have been used to measure the temperature dependence of the spin-lattice relaxation time T(,1) of dangling bond electrons in hydrogenated amorphous silicon and silicon carbide films (a-Si:H and a-Si-C:H) prepared by reactive radiofrequency sputtering. T(,1) determinations were made using absorption mode saturation technique. The microwave radiofrequency field H(,1) was calibrated by an independent determination of T(,1) using the periodic adiabatic passage technique;Measurements made over the temperature range 100-400K showed T(,1) (PROPORTIONAL) T('-2) behavior. This result is closely similar to that known to apply to a-Si based materials prepared by plasma decomposition of SiH(,4) and by electron-beam evaporation methods, indicating that the T(,1) (PROPORTIONAL) T('-2) behavior of the paramagnetic electrons in such materials is independent of the preparation method;Measurements in the 100-400K temperature range are valuable for distinguishing among possible spin-lattice relaxation mechanisms. At these temperatures, dominance of the direct process appears most unlikely. Therefore, phonon-bottlenecking caused by oxygen contamination of the sample surface, which in conjunction with the direct process can lead to T(,1) (PROPORTIONAL) T('-2) dependence, can be ruled out. Rather, the two-level system (TLS) relaxation mechanism which applies generally to amorphous materials and which also yields T(,1) (PROPORTIONAL) T('-2) behavior appears to be responsible in the present case. This follows from the fact that the a-Si:H and a-Si-C:H samples remained amorphous over the entire temperature range studied.