The polymerization of isobutyl vinyl ether (IBVE) by SK-500 is first order in monomer and directly proportional to the weight of the sieve. Catalytic activity and the solid surface acidity of SK-500 are maximum when the sieve is precalcined at 200°C for 2 h. Pyridine (≈10−4mol/L) inhibits and hydrogen chloride (≈ 10−4mol/L) cocatalyzes the polymerization. Both the rate and molecular weight increase to a maximum at ≈7% uptake of water (g/g of dry sieve) and thereafter fall to a limiting value. Molecular weights increase as the monomer concentration in-creases up to ≈1.0 mol/L and attain a limiting value. Molecular weights increase with precalcination temperature up to 500°C and decline thereafter. Rate increases steadily with temperature over the range 2-60°C while molecular weights increase only up to 30°C, above which a sharp fall is observed. The initiation of the polymerization appears to involve the Bronsted acid sites on the SK-500 molecular sieves through a proton transfer to the monomer molecule and follows a conventional cationic mechanism.