The polymerization of styrene with the cationic initiator system AlR2Cl/RCl was investigated further. Earlier conclusions [1] were corroborated and expanded by an analysis of the number and weight average molecular weights. Alkyl halides with low (high) R‒Cl bond dissociation energies are efficient (poor) coinitiators of the polymerization of styrene. In the presence of efficient RCl coinitiators, the molecular weights strongly decrease and the molecular weight distributions strongly increase with increasing conversions (Fig. 1). With poor cointiators, only low conversions are obtained even in the presence of large amounts of AlEt2Cl. The data are discussed in terms of a previously proposed mechanism [1].