The polymerization of styrene mediated by a polystyryl dithiobenzoate was studied by electron spin resonance spectroscopy to determine the concentration of the intermediate radical produced by the addition of polystyryl radical to the dithiobenzoate. The polymerization was also followed by dilatometry to estimate the concentration of the growing radical. The results showed that the fragmentation of the intermediate radical is a fast process with a relevant rate constant on the order of 10(4) s(-1) (at 60 degreesC) and that the intermediate radical undergoes the cross-termination with polystyryl radical to form a 3-arm star chain, thus causing a retardation in the rate of polymerization. The rate constant of cross-termination was estimated to be similar to (somewhat smaller than) that of the termination between polystyryl radicals. The formation of the star was evidenced by an independent model experiment.