AbstractDomain structures of solvent‐cast films of A–B–A block copolymers of styrene‐isoprene‐styrene were studied by electron microscopy by use of the OsO4fixation technique. The effects of varying proportions of the two components and the casting solvent upon the domain structures were examined. It is concluded that the domain formation mechanism can be discussed like that of A–B block copolymers irrespective of sequence arrangements, as Matsuo et al. suggested, by treating A–B–A or B–A–B block copolymers as A–(1/2)B or B–(1/2)A block copolymers. The five types of fundamental domain structures—spherical domains of component A in a matrix of component B, rodlike domains of A in a matrix of B, alternating lamellae of the two components, rodlike domains of B in a matrix of A, and spherical domains of B in a matrix of A—are achieved mainly by the change of the fractional composition of the two components for a given solvent. One of the most significant features of the A–B–A block copolymers in contrast to the A–B block copolymers, i.e., that A–B–A chains can interconnect two A domains, was explored by investigating (1) the swelling behavior in a solvent that is a good one for isoprene but a nonsolvent for styrene and (2) mechanical behavior above the glass transitio