Absorption and steady-state emission spectra and time-resolved luminescence decays have been recorded for CdS nanoparticles prepared using the reverse-micelle method. Spectra were collected in hexane solution, with the surfactant AOT as stabilizer. The emission spectra of these particles are characteristic of trap-state emissions, being broad and red-shifted from the absorption-edge onset. Quantum yields for CdS particles prepared with w(0)'s of 3-7 (w(0) = [H2O]/[AOT]) are low (-1-4%) and are not size dependent. CdS nanoparticles with w(0) = 4 were irradiated at the maximum of the first absorption peak. Irradiation resulted in a red shift and overall decrease in intensity of the absorption spectra, a red shift and increase in intensity of the emission spectra, an increase in the apparent lifetimes, and an increase in the quantum efficiency. Final quantum yields range from similar to9 to 13%.