The effects of liquid-liquid (L-L) phase separation on the crystallization behavior of binary syndiotactic polypropylene (sPP) and ethylene-propylene random copolymer (PEP) mixtures are examined by phase-contrast microscopy (PCM), differential scanning calorimetry (DSC), and cloud point measurements. The PCM experiments reveal that blends of sPP and PEP exhibit a lower critical solution temperature behavior in the melt. The L-L phase diagram, constructed in terms of temperature (T) and composition by cloud point measurements, follows the prediction of the Flory-Huggins theory with the interaction parameter between sPP and PEP [chi(T) = 0.01153 - 4.5738/T (K)]. When the blends are melted within the two liquid-phase (alpha and beta) regions, because of the fact that each phase domain reaches the equilibrium concentration phi(PEP)(alpha) and phi(PEP)(beta) as well as the phase volume fraction nu(alpha) and nu(beta), the crystallinity of each component obeys the equation X-C,X-I = nu(alpha) X-C,I(alpha) + nu(beta) X-C,I(beta), I = PEP, sPP. Also, the equilibrium melting temperatures of both components remain constants, slightly lower than those of neat polymers. For the sPP/PEP blends crystallized from one homogeneous phase in the melt, we observe that the crystallizability of the major component is not greatly affected upon blending. However, the crystallization behavior of the minority component in the presence of the major component is strongly dependent on the crystallization temperature (T-c). When T-c is high, because the decreasing degree of the minority mobility is much greater than the increasing degree of the formed nuclei, the crystallizability of the minor component is depressed significantly. On the other hand, the promotion of the minority crystallizability in the intermediate regime of T-c is mainly because of the large increase of the heterogeneous nuclei upon blending with a major component. (C) 2004 Wiley Periodicals, Inc.