Photopolymerization of methacrylated dental resins at room temperature leads to the appearance of long-life free radicals because of system vitrification. These free radicals were observed by electronic paramagnetic resonance (EPR), and their characterization was undertaken by reference to the reactional mechanism, from the comparison with the model EPR signal and from theoretical simulation. Overlapping of two EPR signals assigned to two different kinds of radicals because of methacrylate end groups accounted for all the experimental and theoretical results. (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1691-1699, 2003.