Amorphous Ga₂₀S₇₅Sb₅ and Ga₂₀S₄₀Sb₄₀ thin films were prepared by thermal evaporation onto clean glass substrates. Transmittance and reflectance measurements were carried out at 300 K, in the wavelength range 400–800 nm, for the virgin and UV illuminated films. Reversible photodarkening is reported for the first time in those films. The photo-induced shift in the non-direct optical energy gap is determined and discussed in the light of current models of photodarkening. The effect of Sb content on the optical band gap and the shift of the absorption edge is also discussed in terms of the coordination number.