We report optical properties of ZnS/ZnMgS strained-layer single quantum wells. The main photoluminescence (PL) peak from ZnS is attributed to light-hole (LH) free excitons. The emission due to heavy-hole (HH) excitons was also observed. With decreasing well width. these emission lines shift to higher energy owing to quantum confinement. Time-resolved PL spectra suggest rapid relaxation from WH band to LH band in the ZnS well. Localization of excitons in the ZnMgS barrier, which is due to fluctuation of alloy composition, is also suggested. On the other hand, localization of excitons in the ZnS well, caused by the fluctuation of the well width, was hardly observed for the sample with good quality. The LH exciton emission showed fast initial decay time of 16-36 ps, which seems to also depend on the sample quality.