AbstractThe equilibrium in the system containing hexamethyldisiloxane, trifluoroacetic acid, and the inert solvent methylene chloride is studied. The system may serve as a model of reactions which play an important role in the cationic polymerization of cyclic siloxanes. The kinetic studies confirm that analogous reactions control the concentrations of silanol and silyl ester end groups as well as of acid and water during the polymerization. Allowing for some hydrogen bonding formation in the analysis of these equilibria the respective equilibrium constants can be obtained preserving almost constant values in the total range of concentrations studied.