Lyotropic phase morphologies of amphiphilic poly(butadiene-b-ethylene oxide) (PB-PEO) block copolymers are studied using transmission electron microscopy, small-angle X-ray scattering, small-angle neutron scattering, and polarized optical microscopy. The PB-PEO block copolymers form type-1 lyotropic phases comprising disordered micellar solutions (L-1), spheres arranged on a bce lattice (I-1), hexagonally packed cylinders (H-1), and lamellae (L-a). Increasing molecular weight destabilizes the I-1 and H-1 phases and lowers the degree of order. For high molecular weight block copolymers the increase in chain conformational entropy leads to the formation of the sponge phase (L-3) Tile transmission electron micrographs allow a detailed analysis of packing defects and epitaxial relations of the block copolymer lyotropic phases.