The optical reflectivity spectrum of CdIn2S4 is obtained in the photon energy range from 1.549 eV (8,000 Å) to 4.132 eV (3,000 Å). A peak and a shoulder are observed in the spectrum at 3.711 eV (3,340 Å) and 2.660 eV (4,660 Å), respectively. The structure is assigned to the critical points in a bond calculation.