An analysis of the products of sodium-catalyzed polymerization of butadiene and isoprene in the presence of toluene suggests that initiation of polymerization takes place by the formation of a disodium addend rather than a free radical. The diaddend reacts with toluene to yield butene-2 and sodium benzyl. Since all of the low molecular weight polymers analyzed were shown to conform to the general formula C6H5CH2(monomer)nH, sodium benzyl is considered to be the chief chain initiator. Chain termination took place by reaction with toluene to regenerate the chain carrier, sodium benzyl. The nature of the acids separated on carbonation of a polymerizing system supports this conclusion.