The nonisothermal crystallization and morphology of three oxyethylene-oxybutylene block copolymers with different architectures (E50B70, B65E75B65, and E35B114E35) were compared with those of three blends (E56B27/B-144, B37E77B37/B-14, and E38B38E38/B-14) With the same composition and morphology (E and B represent oxyethylene and oxybutylene units, respectively, and the subscripts denote the degree of polymerization), and the effect of the amorphous block was examined. The neat block copolymers had larger d-spacings and higher melting temperatures than the corresponding blends. In nonisothermal crystallization, the neat block copolymers had lower crystallization temperatures at high cooling rates. The difference in the crystallization temperatures became smaller at low cooling rates, and some of the neat block copolymers could have higher crystallization temperatures. Polarized optical microscopy showed that the neat block copolymers had smaller dimensions of crystal growth and smaller size of spherulites than the blends. The lower crystallization temperatures and less perfect morphology were attributed to the slower rate of conformational rearrangement of the amorphous block, which was required by the chain folding of the crystallizable block. This effect was more evident in the E35B114E35 tribtock copolymer, in which both ends of the amorphous B block were immobilized at the interface. (C) 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 870-876, 2004.