The kinetics of the cationic ring-opening polymerization of octamethyl-1,4-dioxatetrasilacyclohexane, 2D2, initiated with trifluoromethanesulfonic acid in methylene chloride were studied. The cyclic oligomer formation, polymer formation, and monomer conversion were simultaneously followed by gas-liquid chromatography. The kinetic results provide evidence of direct interconversion of monomer and lower cyclic oligomers. The correlation of this process with the formation of the polymer fraction may be understood on the assumption that the chain propagation and the cyclic interconversion proceed through the same intermediate. The results are best explained by a mechanism which assumes that tertiary silyloxonium ions of a ring compound are active centers of propagation and at the same time serve as intermediates in the interconversion of cyclics. These cationic centers can undergo ring expansion-ring contraction isomerization which leads to the interconversion of cyclic oligomers. They are mostly formed by an acid activation of reactive end groups. The ester end group itself is unable to propagate the chain, but it may be converted with acid to the oxonium active center.