We investigate the thermorheological properties of a partially miscible polymer blend exhibiting lower critical solution temperature phase diagram and having high glass transition and molecular weight constituents, namely poly(styrene-co-maleic anhydride)/poly(methyl methacrylate), SMA/ PMMA, in the nonlinear regime. By using parallel plate, sliding plate, and capillary rheometry, with the knowledge of the blend＇s phase behavior, we address the following aspects of its dynamic response: (i) the sensitivity of nonlinear viscoelastic properties to the phase behavior, which is reflected in the observed thermorheological complexity and a dual-peak stress response in large amplitude oscillatory shear; (ii) the effects of shear history on the morphological and rheological properties, which corroborate the picture of shear-induced fibrilar-type growth of the SMA-rich inclusions in the phase separated regime; and (iii) the evolution of shear and normal stresses and the relevant morphological changes in the phase separated blend during startup of simple shear flow. These results suggest that the main rheological features of complex partially miscible polymer blends can be understood using ideas from the behavior of immiscible blends, provided that their phase state is precisely known. (C) 2000 The Society of Rheology. [S0148-6055(00)00201-7].