Optical properties of amorphous Se_65−xAs₃₅Sb_x thin films with different compositions (x = 0, 2, 4, 6, 8 and 10 at%) deposited by evaporation technique have been investigated by measuring transmission (T) and reflection (R) in the wavelength range 400–2500 nm. An optical characterization method for uniform films based on Swanepoel’s method has been employed to extract the refractive index n and film thickness d, with high precision (better than 1%). The calculated thickness for all thin films was about 1 µm. In addition, the absorption coefficient was evaluated in the strong absorption region of T and R. The possible optical transition in these films is found to be allowed indirect transition with energy gap  decreases from 1.72 to 1.53 eV with increasing Sb content at expense of Se. The chemical bond approach has been applied to explain the decrease of the optical gap with increasing Sb content …