Phase-contrast microscopy, small-angle light scattering, and fluorescence microscopy have been combined in situ to study domain deformation, breakup, and homogenization in unstable mixtures of polystyrene (PS) and polybutadiene (PB) under shear flow. Close to the critical point, mixing of the two components toward a single homogeneous phase occurs via repeated deformation and fragmentation of minority-phase droplets, and the data are in good agreement with the mode-coupling renormalization-group (MCRG) theory of a simple binary mixture under shear flow. Well into the two-phase region of diluted high-molecular-weight PS/PB blends, however, the data suggest a tendency for T-c((gamma) over dot) to saturate at very high shear rates.