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Changes in the refractive index and thickness of amorphous As2S8 semiconductor film are observed under illumination, annealing–illumination, and annealing-illumination cycles. Properties of the As2S8 film are measured using techniques of prism coupler, x-ray diffraction spectra and Raman spectra. After light illumination the refractive index change in an as-deposited As2S8 film is one order of magnitude greater than that of an annealed As2S8 film, corresponding to ∼0.06 and ∼0.0057, respectively. Photoexpansions with a change in the relative film thickness of the as-deposited and annealed As2S8 film are nearly -3.5% and -2.1%, respectively, which exhibit the photodensification. The full reversibility of the photoinduced refractive index change is claimed on an annealed As2S8 film sample. [ABSTRACT FROM AUTHOR]