The infrared quenching of the green luminescence of ZnS : Cu and the 1.0-μ luminescence of CdS: Cu has been studied. A quenching band at 2.4μ was found for ZnS: Cu in addition to the two well-known bands at 0.8μ and 1.2μ. The new quenching band is identified as hole transition from the e state of Cu 2+ to the valence band. The transient stimulation effect produced by the quenching radiation can be explained as the transfer of electrons from slow centers to fast centers using the unassociated pair model. For CdS: Cu three quenching bands centered around 0.8-0.9, 1.4, and 2.5μ have been observed, and they can be ascribed to hole transitions among the two crystal-field states of Cu 2+ and the valence band. Quenching of photo-Hall effect by radiation of 0.8-0.9 and 1.2-1.3μ has been observed for both ZnS: Cu and CdS: Cu. The reduction of Hall mobility may be interpreted as the result of two-carrier conduction. For CdS : Cu quenching of photoconductivity was also found at 2.5μ.