The results are presented of a study of the electrical and optical properties of vacuum evaporated amorphous thin films in the S0.8Se16M0.2 (M = Al, Ag or Cu) system. The activation energy and the pre-exponential factor which appear in the do conductivity are found to be higher in case of Cu than in case of Ag and Al. The reflectance and transmission are used to measure the optical gap. The glass S0.8Se16Cu0.2 behaves as a quasi intrinsic semiconductor (the electrical activation energy is about half of the optical gap). The electrical activation energy is about one-third of the optical gap for the chalcogenide glasses S0.8Se16Al0.2 and S0.8Se16Ag0.2. The variation in the refractive index and the imaginary part of the dielectric constant with photon energy have also been reported. The influence of composition on the investigated parameters is reported. (C) 2003 Elsevier Science B.V. All rights reserved.