Red luminescence in CdS has been studied in as‐grown and ion‐implanted crystals. An IR band consisting of two emission bands, IR1and IR2, is produced by ion implantation. The higher‐energy band, IR1, has the same origin as the R band observed in some as‐grown crystals. A comparison of the R and IR1bands in CdS with self‐activated (SA) luminescence in ZnS and GaAs shows that they are SA luminescence at 1.7 eV at low temperatures in CdS. The temperature variations in peak energy, half‐width, and intensity of luminescence are discussed, and the behavior is explained in terms of a configurational coordinate model. A vibrational energy of 0.025 eV, which is about 70% of the LO phonon energy in CdS, is calculated for the excited state of the center. The study of the excitation dependence of the center shows that the luminescence is due to an electronic transition within a localized center and not due to a donor‐acceptor pair recombination. The center has an excitation band about 0.22 eV below the band gap. The annealing behaviors of the IR1and IR2centers are similar to each other, the bands abruptly disappear over 400°C. There are competitive phenomena between the IR1and IR2centers as to temperature and excitation‐intensity dependences. We tentatively attribute the IR2band to an aggregation defect center.