The electrical and optical properties of diamond-like amorphous carbon films, prepared via the dc glow discharge decomposition of acetylene, show a remarkable dependence on preparation conditions. In particular, as a function of the substrate deposition temperature Td, the room temperature electrical conductivity of the films increases from 10 to the minus 16th power to 10 to the minus 6th power/ohm/cm and the optical energy gap decreases from 2.1 to 0.9eV as Td is increased from 25 to 375 C. The films would have essentially graphitic electrical and optical properties when deposited at Td> or = 400C. For possible semiconductor applications, we have shown that these diamond-like films can be doped n- or p-type via incorporation of P or B atoms during deposition. This doping effect has been confirmed via thermopower measurements. At present we are studying the thermal stability of these films. Results obtained from a study of the effects of annealing on the optical properties of these films will be presented.