Samples of polystyrene, prepared using 14C-tetraethyl thiuram disulphide, have been treated with various chemical reagents. The specific activities of the polymers either fall to zero or are unaffected by the treatments, suggesting that the polymers contain only one type of labelled fragment. The results of the tracer experiments have been combined with kinetic observations and with measurements of molecular weights. It is concluded that the thiuram compound initiates polymerization through the agency of the radical (C2H5)2N.CS.S˙ and that this radical is produced also during transfer to initiator. The retardation evident at higher concentrations of the additive is attributed to primary radical termination and recombination. Other observations recorded include the effects of the compound upon the polymerization of methyl methacrylate, the sensitivity of the compound to light, and the reactivities as initiators of polymers prepared in its presence.