A study of the electrolyte electroreflectance spectra of CuInS2grown by the traveling‐heater method has been made. The energy gaps and broadening parameters were studied by varying the composition of the samples. Our results show that the energy gaps are in the range of 1.52 to 1.53 eV and have band‐gap narrowing as the deviation from the stoichiometry increases, while the broadening parameters are in the range of 0.040 to 0.060 eV and found to be sensitive to the crystallization of the single crystal.