The effects of adding A-B diblock copolymer to a polymer blend (A/B) on phase-separation kinetics and morphology have been investigated in a fixed shallow-quench condition (DeltaT = 1.5 degreesC) by in situ time-resolved light scattering and phase-contrast optical microscopy. A shear-quench technique was used in this study instead of a conventional temperature-quench method. Mixtures of nearly monodisperse low relative-molecular masses of polybutadiene (M-w = 2.8 kg/mol), polystyrene (M-w = 2.6 kg/mol), and a near-symmetric butadiene-styrene diblock copolymer (M-w = 6.3 kg/mol) as an interfacial modifier were studied. We observed that the addition of the diblock copolymer could either retard or accelerate the phase-separation kinetics depending on the concentration of the diblock copolymer in the homopolymer blends. In contrast to the conventional temperature quench, we observed complex phase-separation kinetics in the intermediate and late stages of phase separation by the shear-quench technique. (C) 2001 John Wiley & Sons, Inc.