Transmittance measurements in the spectral range 300-900 nm were used to study the optical properties of Ge20SbxSe80-x (where 5 less than or equal to x less than or equal to 20 at.%) thin films. It was found that the mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap of the as-deposited films decreases monotonically with increasing antimony content, a result which was interpreted on the basis of the chemical-bond approach proposed by Bicermo and Ovshinsky. Annealing of the Ge20Sb15Se65 films at temperatures higher than 425 K was found to decrease the optical gap. The microstructure of the asdeposited Ge20SbxSe80-x thin films was investigated. Selected area electron diffraction for thermally annealed Ge20Sb15Se65 was used to characterize different phases observed during the crystallization process where the crystalline GeSe2 and Sb2Se3 phases were separated. Increasing the amount of these crystalline phases while increasing the annealing temperature could be possible for the continuous decrease of the optical gap of the Ge20Sb15Se65 film. The effect of antimony content on the high-frequency dielectric constant (e(infinity)) and carrier concentration (N) is also studied. (C) 2002 Elsevier Science B.V. All rights reserved.