A quenching of the exciton‐related blue‐edge photoluminescence was observed in high‐purity single crystals of CdS at their electrical breakdown at 4.2°K. The ionization or destruction of the exciton structures by the carriers in breakdown appeared to be an adequate explanation of this quenching. A differential negative resistance was also seen at breakdown and in some crystals after breakdown as well. This negative resistance was also studied in an attempt to understand the origin of the carriers in breakdown.