The nature of the green and blue emission at 78°K in CdS has been investigated. On the basis of the wavelengths of absorption and emission lines the blue component is assigned as exciton decay. Measurement of the decay of the blue luminescence, following excitation by a 10−8sec pulse of 1‐Mev electrons, gave an exciton lifetime <10−8sec. Similar measurements have shown that the green component has a slower decay with emission occurring as long as 20 &mgr;sec after the excitation has ended. These results have been used to support the recombination of a free electron with a trapped hole as the mechanism for green edge emission. Heating in sulfur vapor quenches the green luminescence in a controllable fashion. From the infrared reflection spectra of CdS in the region 10 to 50 &mgr; the optical phonon frequencies were determined. The value of 305 cm−1for the longitudinal phonon is in good agreement with the prediction of Kro¨ger and Meyer from edge emission measurements, showing that the emission is coupled to the lattice through the longitudinal optical phonon. Based on an analysis of the effects on edge emission produced by heat treatment in sulfur vapor and electron irradiation with 0.20 and 1.0‐Mev electrons, a sulfur vacancy is suggested as the recombination center associated with the green edge emission.