The phase-separation behavior of poly(methyl methaerylate)/poly(alpha-methyl styrene-co-acrylonitrile) (PMMA/alpha-MSAN) blends upon heating was studied through dynamic rheological measurements and time-resolved small angle light scattering, as a function of temperatures and heating rates. The spinodal temperatures could be obtained by an examination of the anomalous critical viscoelastic properties in the vicinity of phase-separation induced by the enhanced concentration fluctuation on the basis of the mean field theory. It is found that the dependence of the critical temperatures determined by dynamic rheological measurements and small angle light scattering on heating rates both deviates obviously from the linearity, even at the very low heating rates. Furthermore, the cloud-point curves decrease gradually with the decrease of heating rates and present the trend of approaching T(g)s of the blends. The nonlinear dependence is in consistence with that extracted from the isothermal phase-separation behavior as reported in our previous paper. It is suggested that the equilibrium phase-separation temperature could be hardly established by the linear extrapolating to zero in the plotting of cloud points versus heating rates. (c) 2006 Wiley Periodicals, Inc.