Three heteroaromatic thiols (2-mercaptobenzothiazole, 2-mercaptobenzoxazole and 2-mercaptobenzimidazole) were investigated in regard to their abilities as a co-initiator in free radical polymerization induced by cyanine dye/borate salt photoinitiating system. The kinetic results shown that the new three-component photoinitiating systems are more effective than two-component ones. The mechanism of photochemical and chemical processes occurring during the irradiation of the three-component initiators was proposed on the basis of the laser flash photolysis. The addition to the cyanine dye borate salt of a second co-initiator (thiol) results in the generation besides butyl radical the second free radical which can start the polymerization chain reaction. (C) 2010 Elsevier Ltd. All rights reserved.