A study has been carried out on the preparation of block copolymers of styrenedimethylsiloxane and isoprene-dimethylsiloxane. This has been accomplished by means of anionic polymerization, using potassium metal or potassium naphthalene as initiator, in tetrahydrofuran. Block copolymers of the ABA type were synthesized by first preparing the polystyrene or polyisoprene dianion, followed by polymerization of the octamethylcyclotetraeiloxane. Proof of the presence of these block copolymers was obtained from their solubility behavior and from the ability of the isoprene-siloxane polymers to undergo vulcanization by sulfur. The chain length of the polymeric blocks could be controlled in a predictable fashion, leading to a central segment (B) of polystyrene or polyisoprene, having a narrow molecular weight distribution, and two end segments (A) of polydimethylsiloxane, having a most probable distribution. The reverse type of block copolymer was not possible due to the inability of the siloxane to initiate styrene or isoprene polymerization.