In previous investigations it has been shown that infra-red enhancement of electroluminescence in zinc sulphide is due to the release of positive holes, which migrate to the cathode potential barrier and are trapped there in surface states, producting a greater influx of electrons to excite the crystal. The present paper describes techniques developed to measure the positive-hole lifetimes in the surface states and their dependence on barrier potential and on the presence of background radiation producing electrons. Lifetimes of 5-10 ms are found, decreasing as the cathode is made more positive. The analysis of data obtained throws light on other and independent observations of electroluminescence in zinc sulphide.