The kinetics of the cationic ring-opening polymerizations of epoxide monomers were controlled through the use of supramolecular proton complexes. α,ω-Diglycidyl oligoethylene oxides bearing multiple ethyleneoxy spacer groups form metastable supramolecular proton complexes with strong Brønsted acids generated either by the photolysis of onium salts or by their redox reactions with reducing agents. Trapping of the Brønsted acids by in situ complex formation results in a delay of the onset of cationic ring-opening. However, once polymerization begins, the epoxide groups of the monomer are very rapidly consumed resulting in characteristically highly exothermic autoaccelerated polymerization reactions. Crown ethers can also form supramolecular complexes with hydronium ions derived from the reaction of protonic acids with water and these complexes can be used to modify the kinetic behavior of the cationic ring-opening thermal and photopolymerizations of a variety of epoxide monomers. The ring size of a crown ether has a strong influence on the stability of the supramolecular complex formed, and this was shown to have a major impact on the kinetics of the cationic ring-opening polymerizations.