We explore the dynamic response of a marginally entangled poly(dimethyl siloxane)/poly(ethyl methyl siloxane) blend in simple shear flow using theology and small angle light scattering in situ. Data are collected in both the flow/gradient and flow/vorticity planes. We find that for this model binary blend, exhibiting upper critical solution temperature behavior, shear suppresses the large pretransitional concentration fluctuations and eventually yields homogenization. The simple modified Cahn-Hilliard-Cook phenomenological model of Lai and Fuller [J. Polym. Sci. Part B: Polym. Phys. 32, 2461-2474 (1994)] describes semiquantitatively the time evolution of the structure factor and orientation angle well, supporting the experimental observations of enhanced orientation with flow. In the phase separated region, when the late stages of spinodal decomposition, are reached, the shear-induced anisotropic distortion of the spinodal ring, orienting in the flow direction, and its relaxation upon flow cessation are probed and found to depend on the distance from the spinodal point. At intermediate times the spinodal ring gives rise to unstable butterfly patterns, as a consequence of the coupling of fluctuations with the flow, yielding elongated droplets at later times. The latter turn into streaks at higher rates, suggesting that the mechanism of shear-induced homogenization from the spinodal region, is analogous to that observed in blend solutions. A kinetic phase diagram indicating different microstructures at different temperatures and shear rates, is constructed. These results underline important similarities between binary polymer blends and polymer blend solutions and mark the important parameter ranges for controlling their theology. (C) 2003 The Society of Rheology.