Amorphous Cd20Se80-xMx (M: Zn or In or Sn and x=0 & 10) thin films were deposited on glass substrates by thermal evaporation. The spectral dependence of the absorption coefficient was determined from the transmittance and reflectance spectra, in the wavelength range 200-2500 nm. Both direct and indirect electronic transitions were responsible for the optical properties of Cd20Se80 thin films while only the direct allowed transition was active for the other films alloys. The variation in the optical band gap (Eg) with the addition of the third element was discussed in terms of the width of localized states (Ee) and chemical bond approach model (CBA). The optical constants (refractive index (n) and absorption index (k)) of the films were studied and the dispersion of the refractive index was discussed using the Wemple-DiDomenico single oscillator model. The real and imaginary parts of the dielectric constant in addition to the volume (VELF) and surface (SELF) energy loss functions of the films were also determined. Generally, the addition of Zn was found to have the opposite effect to that of In or Sn addition.