Functionalized silica nanoparticles with thiol or acrylate groups were synthesized, and the effect of the functionalized particles on the thiolene photopolymerization kinetics was investigated by real-time FTIR spectroscopy. To explain the effect of the nanoparticles on thiol-ene polymerization kinetics, the thiol-ene polymerization at the interface of the particle and the bulk monomer was studied in conjunction with the effects of light intensity and viscosity changes caused by the nanoparticles. These results are compared with corresponding nanofilled acrylate systems. Nanoparticles were not found to significantly affect the polymerization of acrylate-based nanocomposites regardless of the functional group type attached to the particle surface while significant changes in polymerization kinetics were observed with thiol-ene based nanocomposites. The thiol-ene polymerization rate decreases with increasing particle content for small amounts of particle loadings due to a stoichiometric imbalance of thiol and ene groups at the particle surface. However, the polymerization rates increase with larger particle loadings because of polymerization viscosity enhancements. Thiol-ene based nanocomposites exhibit higher final conversions than acrylate systems, reduce the oxygen inhibition relative to acrylate polymerizations, and still react rapidly to form highly crosslinked, hard, high glass transition temperature materials. (c) 2006 Elsevier Ltd. All rights reserved.