The liquid-liquid phase-separation (LLPS) behavior of poly(n-methyl methacrylimide)/poly(vinylidene fluoride) (PMMI/PVDF) blend was studied by using small-angle laser light scattering (SALLS) and phase contrast microscopy (PCM). The cloud point (T-c) of PMMI/PVDF blend was obtained using SALLS at the heating rate of 1 degrees C min(-1) and it was found that PMMI/PVDF exhibited a low critical solution temperature (LCST) behavior similar to that of PMMA/PVDF. Moreover, T-c of PMMI/PVDF is higher than its melting temperature (T-m) and a large temperature gap between T-c and T-m exists. At the early phase-separation stage, the apparent diffusion coefficient (D-app) and the product (2Mk) of the molecules mobility coefficient (M) and the energy gradient coefficient (k) arising from contributions of composition gradient to the energy for PMMI/PVDF (50/50 wt) blend were calculated on the basis of linearized Cahn-Hilliard-Cook theory. The kinetic results showed that LLPS of PMMI/PVDF blends followed the spinodal decomposition (SD) mechanism. (C) 2008 Wiley Periodicals, Inc.