The ability to deliberately shift the phase diagram (and thus the morphology) of partially miscible polymer blends through the application of flow is of particular interest in manufacturing, since it opens up a range of opportunities for the generation of products with ''controllably inhomogeneous'' microstructures through intelligent manipulation of the processing conditions. A key part of this is the ability to quantify the effect of flow on the phase diagram. This work presents a theoretical analysis of the problem of coupled flow and thermodynamics based on the concept of an elastic excess stored energy, as well as the results of an experimental programme aimed at determining the parameters needed for the solution of the spinodal equation, for the (PSAN-PMMA) system. Preliminary calculations have indicated that the spinodal curve is shifted upwards upon imposition of shear (shear-induced mixing) when the blend viscosity exhibits a negative deviation from the linear mixing rule. In the experimental part, the viscosity of PSAN-PMMA blends at various compositions has been determined and was found to exhibit a negative deviation from additivity. The quiescent phase diagram of the blend was also determined from cloud point measurements using light scattering and constructed through the use of an equation of state. Solutions of the spinodal equation under shear, based on the modified (through a scaling factor) excess stored energy and using experimentally determined rheological and thermodynamic parameters, predict the occurrence of shear-induced mixing for the PSAN-PMMA system. This is in qualitative agreement with experimental evidence and indicates that near-equilibrium thermodynamics and the concept of stored energy may be useful in describing the effect of shear on the phase diagram of partially miscible polymer blends.