High refractive index optical compositions based on polymer matrices filled with high concentrations of ZnS nanoparticles were developed. These materials have good optical properties and processability like usual polymers, and they are suitable for factory scale use. At 25 vol % ZnS nanoparticles concentration an increase in refractive index up to 0.25 in 150 mu m transparent film was obtained. A process was developed for formation of polymeric compositions consisting of polymer filled with high concentrations of inorganic crystalline nanoparticles. Effects connected to high concentration of nanoparticles in the polymer were discussed. Optical properties of these materials were investigated. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1857-1866, 2010