A polymeric surfactant based on carboxymethyl cellulose and alkyl poly(etheroxy) acrylate (CMC-A9) was used in ultrasonically irradiated emulsion polymerization of styrene. Polystyrene nanoparticles (30-60 nm) with high molecular weight were prepared. Compared with the conventional emulsion polymerization, the introduction of CMC-A9 in the polymerization yielded higher monomer conversion in a shorter time and under lower ultrasonic power output. A single latex particle contained only a few polymer chains with average chain numbers of 2-19. The particle size and polymer chain per particle could be controlled by changing the concentration of CMC-A9 and SDS. The effect of CMC-A9 and SDS on ultrasonically irradiated emulsion polymerization kinetics was studied. The results indicated that the ultrasonically irradiated emulsion polymerization had a similar nucleation mechanism to a miniemulsion polymerization system. This is attributed to the high ratio of radicals and a great number of smaller sizes of monomer droplets produced under ultrasonic irradiation. (C) 2004 Wiley Periodicals, Inc.