The photopolymerization of multifunctional thiol (trimethylolpropane tris(3-mercaptopropionate))/vinyl acrylate mixtures as a function of thiol content in the presence and absence of photoinitiators has been investigated using real-time FTIR, thin-film calorimetry, and 2D-COSY NMR. The addition of a multifunctional thiol to vinyl acrylate significantly enhances the conversion of the vinyl double bonds of vinyl acrylate due to the preferential addition of the thiyl radical to the vinyl group. Two separate free-radical polymerization processes, acrylate homopolymerization and thiol/vinyl ester copolymerization, occur simultaneously in thiol/vinyl acrylate mixtures. Thin-film calorimetry and 2D-COSY NMR results are consistent with real-time FTIR results, giving strong evidence for two simultaneous free-radical processes.