Different compositions of amorphous Ge20Se80 and Ge20Se75M5 (M=Cd, Bi, or Zn) semiconducting films were deposited onto cleaned glass substrates by thermal evaporation method. The interference transmission spectra T(λ) at normal incidence for Ge20Se75M5 thin films were obtained in the wavelength range 400- 2500 nm. The direct analysis proposed by Swanepoel that is based on the use of the extremes of the interference fringes was used in order to derive the film thickness and the two parts of the complex refractive index (real part, n, and imaginary part, k,). The dispersion of n is discussed in terms of the Wemple-DiDomenico single-oscillator model. Furthermore, the optical band gap, Eg , has been determined from the absorption coefficient values using Tauc,s procedure. The obtained results of Eg were discussed in terms of the width of localized state Ee.