We have grown high-quality AgGaS 2 single crystals with a chalcopyrite structure. Using an optical absorption measurement, the energy band gap was determined by means of the Elliott-Toyozawa model. The anomalous variation of energy band gap at T < 100K is discussed in conjunction with AgGaSe 2 compound displaying the same behavior. The exponentially increasing absorption tail was explained in terms of a Urbach tail in the temperature range of 100–300K. Characteristic Urbach parameters, including the steepness parameter and Urbach energy as a function of temperature, as well as effective phonon energy ħω, were obtained. The steepness of the absorption spectra is dominated by the effect of the thermal disorder rather than the structural disorder and/or the deviation from the stoichiometry.