The fundamental absorption edge of glassy alloys of an As–Sb–S system was studied in the temperature range 77–300 K. The spectral and thermal behavior of the fundamental absorption edge of As40–x SbxS60 glasses was shown to be described by an exponential function of phonon energy. The temperature and concentration behavior of the optical band gap in the studied glasses was established. The well-known Varshni relationship was used to describe the temperature dependence of the optical band gap.