Hypothesis: Dewetting pathways, kinetics and morphologies of thin films of phase separating polymer blends are governed by the relative mobilities of the two components. Experiments: We characterize the morphological transformations of the nanostructures of a PS/PMMA blend by annealing in toluene and chloroform vapors. Toluene leads to faster reorganization of PS, whereas chloroform engenders the opposite effect. Findings: Spin coating produces a very rough PMMA rich layer that completely wets the substrate and forms a plethora of slender columns protruding through the continuous PS rich layer on top. The nano-structures were stable under long thermal annealing but in the vapor annealing, phase separation and dewetting occurred readily to form the equilibrium structures of dewetted droplets of PS on top of PMMA which also climbed around the PS droplets to form rims. Toluene and chloroform annealing required around 50 h and 1 h respectively to attain the equilibrium. Substantial differences are observed in the intermediate morphologies (heights of nanostructures, roughness and size). PMMA columns remained embedded in the dewetted PS droplets, whereas a high mobility of PMMA in chloroform allowed its rapid evacuation during dewetting to produce an intermediate swiss-cheese like morphology of PS domains. (C) 2015 Elsevier Inc. All rights reserved.