Different photoinitiators based on Barton thiohydroxamic esters, O-acyl-N-hydroxy-pyridine-2(1H)-thione and O-acyl-N-hydroxy-thiazole-2(3H)-thione derivatives, were tested in photopolymerization reactions through time-resolved FTIR spectroscopy. Good rates of polymerization and final monomer conversion were obtained for some compounds. The excited state processes, investigated by time resolved absorption spectroscopy, lie on a fast singlet state cleavage leading to thiyl and alkyl radicals. Both the pyridine-2(1H)-thiyl and alkyl radicals are able to initiate a polymerization, in contrast with the thiazole-2(3H)-thiyl radical. A triplet state is observed for some derivatives. Computational studies help to describe the excited state properties and show a strong difference in the spin localization in the formed thiyl initiating radicals. (C) 2008 Wiley Periodicals, Inc.