Thin films of zinc sulfide (ZnS) doped with Mn, were deposited using magnetron sputter source. The electroluminescent properties of the as-deposited films were relatively poor. Post- deposition rapid thermal annealing (RTA) with and without co-dopants was studied. Significant changes in microstructure and EL performance were observed on the samples after post-sputter processing. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were employed to characterize the microstructure. Both inter-grain and intra-grain distributions of the co-dopants were measured using energy dispersive X-ray spectra (EDX) and the distribution versus thickness was determined by dynamic secondary ion mass spectrometry (SIMS). A correlation between electroluminescent properties and the microstructure is obtained.