We systematically compare the morphological evolution of thin PS/PMMA films, with varying compositions, during thermal annealing on preferential and non-preferential surfaces. On native silicon oxide surfaces, the phase evolution in the films was dictated by the preferential substrate-wetting of PMMA. However, the resulting PS relief structures on the PMMA wetting layer varied with the blend composition, and transitioned from capillary-fluctuation-mediated breakup to random nucleation with increased PS concentration. In contrast, the morphological evolution of the PS/PMMA films on non-preferential surfaces was also dictated by the coarsening of PMMA domain, but proceeded without the formation of a PMMA wetting layer. Both the PS and PMMA domains maintain direct contact with both the substrate and free surfaces throughout the evolution of the morphology. The morphologies at the interfaces were highly correlated, but with distinctive length scales. A variety of dispersed and continuous phase-separated structures can thus be obtained. (C) 2012 Elsevier Ltd. All rights reserved.