Changes in some physical and optical properties of thermally evaporated thin films from the systems As-S and As-S-Tl over a wide range of concentrations have been investigated. The influence of the conditions of vacuum deposition and light exposure has been demonstrated. The optical transmission and reflection of thin layers deposited on BK-7 optical substrates have been measured in the spectral region of 350-2000 nm and the linear (n) and nonlinear (n2) refractive indices and optical band-gap, E-g, as well as the oscillator fitting constants, were calculated. Boling's formula is used to predict n2 from the dispersion and the magnitude of n. Data for changes in the glass-transition temperature, T-g, microhardness, and rate of dry etching of thin chalcogenide films before and after exposure to light are presented. The addition of Tl in As2S3 leads to an increase in the refractive index and decrease in the optical band-gap. After illumination a photodarkening or photobleaching effect was observed depending on the evaporation conditions. Some of the layers change their etching rate in a plasma which make them suitable for practical applications. Conclusions on the homogeneity of the layers and the origin of photostructural changes in them are drawn. (C) 2004 Kluwer Academic Publishers.