Efficient and photostable ZnS-passivated CdS:Mn (CdS:Mn/ZnS core/shell) nanocrystals were, synthesized using reverse micelle chemistry. CdS:Mn/ZnS core/shell nanocrystals exhibited much improved luminescent properties (quantum yield and photostability) over organically (n-dodecanethiol-) capped CdS:Mn nanocrystals. This is the result of effective, robust passivation of US surface states by the ZnS shell and consequent suppression of non-radiative recombination transitions. The dependence of photoluminescence (PL) intensity has been observed as a function of UV irradiation time for both organically and inorganically capped CdS:Mn nanocrystals. Whereas organically capped CdS:Mn nanocrystals exhibit a significant reduction of PL intensity, CdS:Mn/ZnS core/shell nanocrystals exhibit an increased PL intensity with UV irradiation. XPS (X-ray photoelectron spectroscopy) studies reveal that UV irradiation of CdS:Mn/ZnS nanocrystals in air atmosphere induces the photo-oxidation of the ZnS shell surface, leading to the formation of ZnSO4. This photo-oxidation product is presumably responsible for the enhanced PL emission, serving as a passivating layer.