ZnS-encapsulated spherical SiO₂ nanostructures were prepared by a one-step solvothermal method. The samples were characterized by x-ray diffraction, transmission electron microscopy, selected-area electron diffraction, infrared spectra, UV–visible spectra, and photoluminescence. It is found that 27 nm ZnS in a wurtzite phase was encapsulated by a low-porosity SiO₂ layer, which compares to a particle size of 8.5 nm for bare ZnS as prepared under similar conditions. Contrary to the theoretical predictions of the quantum size effect, the encapsulated ZnS showed a band gap energy surprisingly larger than that for the bare one. Moreover, the encapsulated ZnS/SiO₂ nanostructures exhibited 'self-activated centers', as characterized by a blue photoluminescence emission band at 430 nm, while the bare ZnS nanoparticles gave a surface sulfur species-mediated green emission at 520 nm. These observations are explained in terms of the encapsulated nanostructure and the relevant defect nature in which the SiO₂ layer reduces the nonradiative recombination centers and enhances the photoluminescence intensity.