Poly(ethylene terephthalate) (PET)/poly(ether imide) (PEI) blends were miscible in the melt, but exhibited simultaneous liquid-liquid phase separation and crystallization over a wide range of temperature and composition. The interplay between these two processes is expected to dominate the morphological formation in the blends. In this study, the phase diagram of PET/PEI blend was determined to evaluate the envelop within which liquid-liquid phase separation was operative with crystallization. A UCST phase diagram below 240 degrees C was identified for this system. The effect of liquid-liquid phase separation on the growth of PET spherulites was studied by small-angle light scattering (SALS). Nonlinear spherulite growths were observed for the blends at higher crystallization temperatures of 210 degrees C and 220 degrees C, while the growths were basically linear below 210 degrees C. The nonlinear growth behaviour was discussed based on the competition between spherulite growth and spinodal decomposition.