We have studied the effect of liquid-liquid phase separation on crystallization in near-critical blends of poly(ethylene-cohexene)/poly(ethylene-co-octene) and poly(ethylene-co-hexene)/poly(ethylene-co-butene) using optical microscopy and simultaneous small and wide angle X-ray scattering. Two quenching schemes were used in this study: (1) single-quench, a homogeneous melt quickly cooled to the crystallization temperature, and (2) double-quench, a homogeneous melt quickly cooled to an intermediate temperature, which allows for occurring phase separation but not crystallization, then to the crystallization temperature. We could found more crystalline nuclei in case of single-quench than of double-quench. The long spacing of lamellar crystals is approximately 20 A larger in single-quench than in double-quench, due primarily to the inclusion of more non-crystallizable components in the amorphous layers of lamellar stacks in the former. The degree of crystallinity is about three times higher in single-quench. (C) 2004 Elsevier Ltd. All rights reserved.