Combinations of a dialkylmagnesium and a cerium tert-butoxide complex have been explored for styrene polymerization. Cerium(IV) precursors, equivalently Ce(OtBu)4(THF)x (1) or Ce3(OtBu)10O (2), when associated to 5 or more equivalents of n-butylethylmagnesium (BEM), oligomerize styrene in a controlled way with activities ranging from 10 to 1000 mol St mol Ce−1 h−1 at 25 and 105 ◦C, respectively. Effective chain transfer to BEM takes place in the course of polymerization, as evidenced by the decrease of the molecular weight of polystyrene with increasing amounts of BEM, eventually yielding Et and n-Bu end-capped oligostyrenes (identified by MALDI-TOF-MS) with Mn = 650–7400 g mol−1 and Mw/Mn = 1.3–2.5. ESR analyses established the formation of butyl and/or styryl radicals from these binary systems. The use of phenothiazine as a radical inhibitor did not inhibit completely the polymerization of styrene but significantly decreased the polymer yields. Therefore, the polymerization of styrene is proposed to arise from two different, concomitant pathways, which both enable effective transfer of growing polystyryl chains to dialkylmagnesium species: (i) radical polymerization initiated by free radicals generated from the redox-active metal precursors, as the major pathway, and (ii) coordination/insertion polymerization based on in situ produced Ce(III)- and/or Ce(IV)-alkyl species.