A controlled cationic polymerization of styrene was achieved with an alcohol [water-vinyl monomer adduct: CH3CR(Ph)OH; -R = -H (2), -CH3 (3)] as an initiator coupled with BF3OEt2. In the presence of a fairly large amount of water, the polymerization gave polymers with molecular weights that increased in direct proportion to monomer conversion and agreed well with the calculated values, assuming that one initiator molecule generates one polymer chain, although the molecular weight distributions (MWDs) were broad (M-w/M-n similar to 2). 2,6-Di-tert-butyl-4-methylpyridine (DTBMP) and tetrabutylammounium hydroxide (n-Bu4NOH) retarded the polymerization and narrowed the MWDs. H-1 NMR and MALDI-TOF-MS analysis supported the polymerization mechanism where BF3OEt2 selectively and reversibly activates the terminal C-OH bond derived from the initiator to generate the growing cationic species. Similar controlled polymerizations of p-chlorostyrene (pClSt) and p-methylstyrene (pMS) were also achieved by the R-OH/BF3OEt2 system with a proper ratio of water and DTBMP. This is the first example of a controlled cationic polymerization of styrene via reversible activation of the C-OH bond mediated by BF3OEt2 in the presence of water.