A metal-free initiating system for the living cationic polymerization of alkyl vinyl ethers (VEs) and styrene derivatives was developed using diaryliodonium salts as an organic Lewis acid catalyst. Unlike many past examples of their use as a photoinitiator, diaryliodonium salts were demonstrated to function as a Lewis acid catalyst for cationic polymerization. The cationic polymerization of isobutyl VE smoothly proceeded using a diaryliodonium salt in conjunction with a cationogen that generates a carbon−halogen propagating end, yielding polymers with predictable molecular weights and very narrow molecular weight distributions. The central iodine atom of the diaryliodonium salt most likely exhibited Lewis acidity, thereby generating a dormant−active equilibrium through the reversible generation of a carbocation via the abstraction of the halogen anion from the propagating end. The role of the diaryliodonium salts as Lewis acid catalysts was confirmed by a series of experiments that focused on the effects of concentrations, substituents on the aryl rings, and counteranions. Polymerizations of styrene derivatives and VEs with polar groups also proceeded using diaryliodonium salts in a controlled manner.