The kinetics of the polymerization of hexamethylcyclotrisiloxane D3 initiated by lithium 2,4,4-trimethyl-1-pentanolate (CH3)3CCH2CH(CH3)CH2OLi in THF solution in the range 0–60°C has been investigated. Polymerization was essentially first order in respect to monomer. Independent of the temperature in the range from 0 to 60°C, the external order of the rate with respect to initiator changed from 0.25 to 0.33 by changing the initiator concentration from 10−1 M to 10−3 M. This change indicates a change in the degree of aggregation from 4 to 3 of the propagation-active siloxanolate groups. The presence of aggregates and the change in their degree of aggregation from 4 to 3 has been independently and quantitatively confirmed by viscometry carried out before and after quenching the living D3 polymerization by Men−4SiCln (n=1–4) and Cl2SiMeCH2CH2MeSiCl2. Evidently most of the siloxanolate end groups are organized into aggregates whose size is controlled by the concentrations of the siloxanolate groups in the range from 10−3 to 10−1 M but insignificantly by the temperature from 0 to 60°C. Propagation is due to a relatively small proportion of active unaggregated ion pairs.