Aqueous sulfonic acids (HOSO2R; R = CH3, Ph-p-CH3, and Ph-p-NO2), coupled with a water-tolerant Lewis acid, ytterbium triflate [Yb(OM)(3); OTf = -OSO2CF3], initiate the cationic suspension polymerization of p-methoxystyrene (pMOS) in heterogeneous aqueous media. They induce controlled polymerization of pMOS at 30 degrees C, and the molecular weights of the polymers (weight-average molecular weight/number-average molecular weight similar to 1.7) increase with conversion. These sus pension polymerizations are initiated by the entry of sulfonic acid from the aqueous phase into the organic phase and proceed via reversible activation of the sulfonyl terminus by the Lewis acid.