AbstractThe kinetics of polycondensation of tetramethyldisiloxane 1,3‐diol (HD2OH) have been followed by stopping the reaction at increasing times (by neutralization of the acid and trimethylsilylation of the silanol end‐groups). Analyses of the linear and cyclic oligomers and of the higher polymer have been made by gas–liquid chromatography and by steric exclusion chromatography. At the beginning of the reaction (e. g. between 1 min and 1 h) the main products are linear oligomer H(D)2nOH and D4(formed by cyclization of HD4OH). There are only very small amounts of large cycles. Linear high polymer is then formed slowly together with increasing amounts of cycles D5, D6, D7, …, formed by backbiting reactions. After 24h, the yield of D4is 40–50% and that of linear polymer 40–30% (Mpeak× 104–105). With disilanol concentration 0.45–0.9 mol liter−1and CF3SO3H concentration about 10−3mol liter−1, the reaction is very rapid during 1–2 min and the medium becomes heterogenous with the formation of an inverse water emulsion. The reaction then slows down considerably, most of the hydrated acid being trapped in the water droplets. Silanol consumption is second order in [SiOH] and first order [TfOH]o. Polymerizations made in polar and non‐polar solvents have similar rates, which however increase moderately in the order toluene<1,2‐dichloro‐ethane