The photoluminescence from Mn2+-doped ZnS (ZnS:Mn) nanoparticles prepared in an AOT/isooctane reverse micellar system was investigated. The composition (Mn content) and photoluminescent properties of the nanoparticles depend on the composition of the feed solution and the water content (W-o) of reverse micellar solution. The emission intensity from Mn2+ ions at 580 nm depends mainly on the Mn content in the nanoparticles rather than the nanoparticle size, when the nanoparticle size is controlled by varying the W-o value at a constant feed composition of Zn and Mn.