AbstractPolymerization of hexamethylcyclotrisiloxane (D3) initiated by trifluoromethanesulfonic acid (triflic acid TfOH) was carried out under vacuum, in methylene chloride solution at 20°C. The reaction rate is first order in [D3] and the constant active sites concentration [P*] varies as [P*] = k · [TfOH] · [D3]. The two main products of the reaction, both formed in similar amount from the beginning, are the dimer D6 and a high‐molecular‐weight polymer (HP). The other cyclic products also formed are the multiples of D3 (D9, D12, …), macrocycles MC (number‐average molecular weight M̄n ≈ 104) and small amounts of D4, D5 … (with [D5] > [D4]). The concentration of HP, MC, D6, D9, D12, … and the M̄n of the high polymer grow proportionally to conversion. The main reaction giving the high polymer and D6 is interpreted as a chain reaction, the growing HP bearing at each end potentially reactive silyl triflates which may be activated by the residual acid. D6 is mainly formed by a special type of back‐biting reaction involving transitory tertiary siloxonium ions. The other cycles D3x and the macrocycles mainly result from cyclization reactions of a second population of growing macromolecules which bear a silyl ester at one end and a nucleophilic silanol group at the other end.