Copolymers of the cyclic ketene acetals, 2-methylene-5,5-dimethyl-1,3-dioxane, 3, (M(1)) with 2-methylene-1,3-dioxolane, 4, (M(2)) Or 2-methylene-1,3-dioxane, 5, (M(2)), were synthesized by cationic copolymerization. An experimental method was designed to study the reactivity of these very reactive and extremely acid sensitive cyclic ketene acetal monomers. The reactivity ratios, calculated using a computer program based on a nonlinear minimization algorithm, were r(1) = 6.36 and r(2) = 1.25 for the copolymerization of 3 with 4, and r(1) = 1.56 and r(2) = 1.42 for the copolymerization of 3 with 5. FTIR and H-1-NMR spectra when combined with the values of r(1) and r(2) showed that these copolymers were formed by a cationic 1,2-polymerization (ring-retained) route. Furthermore the tendency existed to form very short blocks of M(1) or M(2) within the copolymers. Cationic copolymerization of cyclic ketene acetals have the potential to be used for synthesis of novel polymers.