Simultaneous small-angle scattering and in situ dynamic mechanical measurements offer an excellent opportunity to relate the macroscopic dynamical mechanical response of block copolymers and their mesoscopic structural behavior. We use small-angle neutron scattering and rheology to examine the ordered phases and the order-order transitions exhibited by a poly(ethylene-alt-propylene)-poly-(dimethylsiloxane) diblock copolymer. An intermediate structure-very similar to the hexagonal perforated layer (HPL) phase reported in other diblock systems-proves to be metastable, and we study the kinetics and epitaxy of its relaxation to the "gyroid" phase of Ia (3) over bar d symmetry. Likewise we study the relaxation of a supercooled hexagonal phase to the gyroid structure and also observe that the gyroid phase is bypassed in a slow cool from the hexagonal phase to the HPL-like structure. The origin of a typical scattering pattern obtained from a highly oriented crystal structure of a sample in the gyroid phase is investigated and related to real space projections of the gyroid morphology. Synchrotron small-angle X-ray scattering is used for high-resolution studies of the ordered phases. Reversibility of transitions between different mesoscopic structures is studied, especially to and from the gyroid phase, and puzzling patterns of the HPL-Like structure are observed.