Structure development of PMMA/SAN blends in shear flow is studied experimentally using optical microscopy and small-angle light scattering (SALS). The presence of flow promotes highly anisotropic structures, and the objective of this work is to obtain a relation between these anisotropic scattering patterns and the morphology. The aim is to understand the influence of flow on morphology development,by resolving the length scales of the domains parallel and perpendicular to the direction of flow. The diffuse-interface model, based on the Cahn–Hilliard theory, is used to predict the morphology as a function of shear by systematically increasing the shear rate. Special precautions are taken to avoid wall effects, demonstrating the importance of sheared periodic boundary conditions.