Cationic copolymerization of tetrahydrofuran (THF) with ethylene oxide (EO) in the presence of diols proceeds with simultaneous participation of secondary and tertiary oxonium ions. This process comprises therefore some features of the activated monomer (AM) and the active chain end (ACE) mechanisms. The mechanism of copolymer formation was evaluated, and the kinetics of competing reactions involving both secondary and tertiary oxonium ions with nucleophiles present in the system was studied. The apparent rate constants were derived, allowing the estimation of the influence of copolymerization conditions on the composition and microstructure of copolymers.