Poly(hydrogenmethylsiloxane-co-dimethylsiloxane)s of various compositions have been prepared by cationic ring-opening polymerization of octamethylcy-clotetrasiloxane (D-4) and 1,3,5,7-tetramethylcyclotetrasiloxane (D-4(H)) in the presence of hexamethyldisiloxane as end-blocker or by rearrangement of poly(hydrogenmethylsiloxane) in the presence of D-4. These copolymers were examined by high resolution IPI NMR (500.13 MHz) and Si-29 NMR (99.37 MHz) spectroscopies. Triad effects were observed by H-1 and up to heptad effects by Si-29 NMR. The chemical shifts were assigned for these stereosequences. The intensities of the triad signals were used to calculate the quantitative parameters describing the microstructure of the copolymer chains: number-average block length (L) and persistence ratio (eta) The values of these parameters for copolymers prepared in various experimental conditions show that the time necessary for redistribution reactions (backbiting) is much larger than the time required to establish the equilibrium between linear polymer and cyclic oligomers. However, redistribution is fast enough to prevent the formation of block copolymers even in the case of the rearrangement of poly(hydrogenmethylsiloxane) in the presence of D-4.