Homopolymerization of octamethylcyclotetrasiloxane, D-4, and two copolymerizations of this monomer with octaphenylcyclotetrasiloxane, D-4(Ph2), and hexaethylcyclotrisiloxane, D-3(Et2), were investigated by monitoring the courses of their occurrence using three different methods: size exclusion chromatography (SEC), thermal gravimetric analysis (TGA), and Si-29 NMR spectroscopy. The results obtained showed that while homopolymerization of D-4 and its copolymerization with D-3(Et2) occurred in a nearly identical manner, suggesting that reactivities of the two monomers toward the silanolate active species were practically identical, the copolymerization of D-4 with D-4(Ph2) exhibited a distinct initial induction period resulting from two factors: (a) very low solubility of solid D-4(Ph2) in D-4 and (b) significantly higher reactivity of D-4(Ph2) than D-4 toward the dimethylsilanolate. On the basis of these results, we propose a new three-stage mechanism for the copolymerization of D-4 with D-4(Ph2), which includes (a) initial formation of B-A-B triblocks of dimethylsiloxane (A) and diphenylsiloxane (B) segments, followed by (b) subsequent formation of A-B-A-B-A pentablock species, and (c) redistribution of the former multiblocks through a siloxane equilibration reaction into copolymers containing single diphenylsiloxy units separated by extended polydimethylsiloxane segments.