Monodisperse, submicron, spherical, and porous clusters of zinc sulfide (ZnS) nanocrystallites were used as a host material, into which gallium phosphide (GaP) was impregnated. A solution containing a phosphinogallane, (t-Bu)[subscript]2GaP(t-Bu)[subscript]2, and a vapor mixture of GaI and P[subscript]4 were used as precursors for GaP;Powders obtained via organometallic precursors were nanocomposite powders with an average crystallite size of 12 nm and maintained the initial powder morphology of ZnS after heat treatment at 800°C for 24 hours. After hot isostatic pressing, relative densities greater than 95% and ultra-fine microstructures with average grain sizes ranging 16-31 nm were achieved. Nanocomposites showed significant increase in hardness, as high as ~100%, compared to pure ZnS. Powders obtained using vapor phase precursors were mainly solid solutions between ZnS and GaP. The powders were densified to over 98% relative densities by hot isostatic pressing. Both hardness and fracture toughness increased with increasing GaP content.