Thiourea, in presence of different oxidants, activates radical polymerization of various vinyl monomers. ESR spectroscopy of propagating radicals from methyl methacrylate gives the well known 9-line spectrum with sets of five strong and four weak lines because of partially resolved hyperfine splitting due to methyl and methylene hydrogens. Spin trapping experiments with N-benzylidene-t-butyl amine-N-oxide (PEN) show considerable variations of the rate of initiation of different vinyl polymerization processes.