The phase behavior of two partially miscible polymer blends in simple shear flow was monitored on-line by Raman spectroscopy and subsequently with optical light microscopy on the resulting extrudate. Emphasis was placed on investigating the phase behavior in two polymer blend systems of high and low shear viscosity as they flow through a custom built flow cell with slit conduit. Shear flows initiated from one- and two-phase temperatures were monitored in the higher viscosity blend system of poly(α-methyl styrene–co-acrylonitrile) (PαMSAN) and poly(methyl methacrylate) (PMMA). For the second lower viscosity blend of poly(styrene–co-acrylonitrile) (PSAN) and PMMA90, shear flow was conducted at one-phase temperature and various shear stresses. These blends exhibit a lower critical solution temperature (LCST). The chosen blend composition used for PαMSAN/PMMA had the widest miscibility window in view of the high viscosity of the neat polymers, while for PSAN/PMMA90 blend, a composition very close to the LCST was investigated for the shear flow effects. We observed that for a blend composition of PαMSAN/PMMA system, at small temperature gap from the quiescent cloud point curve, the morphological effects due to shear heating overrides other effects. For a lower melt viscosity blend composition of PSAN/PMMA90, shear induced demixing initially occurs at low shear rates, shear-induced mixing regime follows with increasing shear flow prior to the onset of shear heating effects that brings the material to two phase region resulting in creation of locally optically observable demixed regions where the temperature rise due to shear heating across the extrudate are at maximum.