The effect of added block copolymer on the phase separation and morphology evolution in a partially miscible blend of polystyrene and polybutadiene near the critical composition is studied by temperature jump light scattering (TJLS) and transmission electron microscopy (TEM). As block copolymer is added, the phase boundary is shifted to lower temperatures and the phase separation process is slowed dramatically. Since the quench depth greatly affects the rate of phase separation in any blend system, we have used equivalent quench depths by adjusting for the shift in the phase boundary as block copolymer is added. The morphology evolution of these ternary blends was studied by preparing TEM specimens at equivalent shallow quench depths (Delta T = 1.6 degrees C) and allowing each blend mixture to coarsen for the time required to reach a specific constant size, or q-value, using the TJLS data on the kinetics of phase separation. The q-range selected was q similar to 0.003-0.005 nm(-1), which corresponds to a spacing of 1-2 mu m in real space. The combination of light scattering and microscopy techniques more rigorously describes the compatibilization process in these complex ternary systems.