Polycrystalline Si1-xGex alloys with different Ge contents were prepared successfully in evacuated quartz tubes, from which a-Si1-xGex:H thin films were prepared by thermal evaporation. Specimens were deposited by varying preparation conditions such as the deposition temperature (T-d), Ge content (x) and dopant concentration of As and Al. The composition of the alloys and films was determined by energy-dispersive spectroscopy (EDS) and electron spectroscopy for chemical analysis (ESCA). Our experimental data show that the optical energy gap (E-g(opt)) decreases with increasing T-d, x and dopant concentration of As and Al, while the Urbach energy (E-u) increases with the same. The refractive index (n) and the extinction coefficient (kappa) increase with increasing T-d, x and dopant concentration. We found two empirical formulas, one relating E-g(opt) with x and the other describing the dependence of n on x.