Amorphous Gex Se90-x Sb10 thin films were prepared by thermal evaporation under vacuum onto glass substrates. Reflectance and transmittance were measured in the wavelength range 190-900 nm. The optical properties of the as deposited and UV-irradiated films at different exposure times were reported. The compositional dependence of the optical constants (absorption coefficient, the non-direct optical gap Eg, refractive index (n), and the extinction coefficient (k) were evaluated and discussed in terms of the Ge content and the chemical bond network model.