A series of five cyclic block copolymers of styrene and butadiene, having essentially the same molecular weight (52 5 kg/mol) and PS volume fraction varying from It to 70%, were synthesized by cyclization of alpha,omega-dilithium polystyrene-polybutadiene-polystyrene triblock copolymers with his(dimethylchlorosilyl)ethane. The cyclic block copolymers thus obtained have practically the same molecular weight and composition as their corresponding linear triblock copolymers. All materials were investigated via transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) techniques. In three cases where the cyclic and the corresponding linear block copolymer had the same morphology, the domain spacings of the cyclic block copolymers are found to be 84%-89% of those of their respective linear triblock copolymers. In the other two cases different morphologies are found in the cyclic and its corresponding triblock copolymer. Compared to their linear triblocks, the interfaces are curved away From the connected end blocks.