The phase behavior and phase-separation dynamics of polystyrene/polyvinyl methyl ether (PS/PVME) blend with a critical composition of 70 vol % PVME were examined with a light scattering technique under a shear-rate range of 0.1-40 s(-1). If the shear rates were less than 8 s(-1) and the starting temperatures of the measurement were 343 and 383 K, respectively, two cloud points were observed, whereas after the shear rate was higher than 8 s(-1), only one cloud point existed, 20 K higher than that of the static state of the blend. Investigation of the phase-separation dynamics at 443 K suggested that in the vorticity direction the phase-separation behavior at the early stage and the later stage can be explained by Cahn-Hilliard linearized theory and the exponent growth law, respectively. Phase separation occurs after a shearing time, which was called a delay time tau(d).The delayed time tau(d), the apparent diffusion coefficient, and the exponent term of the blend show strong dependence on shear rates. A theoretical prediction of the phase behavior of PS/PVME under a shear flow field by introducing an elastic energy term into Flory's equation-of-state theory was made, and the prediction was consistent with the experimental results. (C) 2003 Wiley Periodicals, Inc.