The cationic polymerization of isoprene using the 1-(4-methoxyphenyl)ethanol (1)/B(C6F5)(3) initiating system in solution (dichloromethane or alpha,alpha,alpha-trifluorotoluene) and in aqueous media. (suspension, dispersion, or emulsion) is reported. In organic solvents the reaction proceeded by controlled initiation via 1, followed by irreversible termination, thus affording polymers with a number-average molar mass M-n <= 5000 g mol(-1) and a molar mass distribution or MMD (M-w/M-n <= 2.5) and rather high content of intact double bonds (>= 70%) in the polymer backbone. In particular, using alpha,alpha,alpha-trifluorotoluene as polymerization solvent in place of dichloromethane promoted the synthesis of polyisoprene chains with M-w/M-n similar to 1.4 and larger content of intact double bonds (up to 88%). In the absence of 1, polyisoprenes with fairly high molar mass (M-n up to 18 000 g mol(-1)) and M-w/M-n < 24 were synthesized through adventitious water/B(C6F5)(3)-initiated cationic polymerization of isoprene. In aqueous media, the cationic polymerization of isoprene with 1/B(C6F5)(3) proceeded without any side reactions (cyclization, branching). However, aqueous conditions afforded only moderate yield (up to 60% monomer conversion) and polyisoprenes with low M-n (<= 1200 g mol(-1)) and M-w/M-n <= 1.7 were obtained. By comparing different characterization data (NMR, mass spectrometry), it was demonstrated that under appropriate conditions (in organic solvents at -30 degrees C or in aqueous media) polyisoprenes carrying the initiator fragment from 1 at the alpha-end and an olefinic terminal group can be produced almost exclusively in a trans-1,4 configuration (92-96.5%).