This study shows that ytterbium triflate [Yb(OTf)(3); OTf = OSO2CF3] induces cationic polymerizations of isobutyl vinyl ether (IBVE) and p-methoxystyrene (pMOS) in aqueous media in conjunction with the hydrogen chloride-vinyl monomer adduct [CH3-CHR-Cl; R = OiBu, C6H4-OCH3] as the initiator at room temperature. These "suspension" polymerizations are carried out by adding neat monomer and then a solution of the initiator into an aqueous solution of Yb(OTf)(3) under vigorous stirring, and the polymerizations occur smoothly without being inhibited by water. Under these conditions, IBVE polymerization was uncontrolled, but pMOS can be polymerized in a controlled fashion where the molecular weight distributions (MWDs) are relatively narrow ((M) over bar(w)/(M) over bar(n) similar to 1.4) and the molecular weights increase in proportion to monomer conversion. The reactions proved to be cationic, being quenched by NaOH and EtOH but not by 1,4-benzoquinone. The aqueous hydrochloric acid/Yb(OTf)(3) initiating system also generates similar long-lived pMOS polymer in water. The success in such controlled cationic polymerizations in aqueous media is due to the stability of Yb(OTf)(3) as well as the dormant C-Cl poly(pMOS) terminal in water.