The emulsion polymerization of some monomers can occur without the conventional free radical initiators under ultrasonic irradiation. However, the initiation mechanism is still under controversy. In this paper, the sources of free radicals arising from ultrasonically initiated emulsion polymerization were investigated. Experimental results show that ionic surfactants play a very important role in obtaining a high polymer yield. While monomer conversion is very low in the absence of surfactants or in the presence of nonionic surfactants, it increases significantly upon addition of a little amount of ionic surfactant. FTIR and a radical trapping experiment confirm that the free radicals involved in the irradiation process originate from the decomposition of the ionic surfactants. Under ultrasonic irradiation, ionic surfactants undergo bond scission between the alkyl and ionic group, where the bond is the weakest along the chain. (c) 2005 Wiley Periodicals, Inc.