The phase behavior of the semicrystalline polymer blend composed of isotactic polypropylene (iPP) and linear low density polyethylene (PE) was studied using small angle X-ray scattering (SAXS) and optical microscopy (OM). Based on the random phase approximation, the iPP/PE interaction parameter, chi was obtained, and used to construct the iPP/PE phase diagram. The chi values reported in this study are lower than the chi values for deuterium-labeled moieties, measured by small angle neutron scattering (SANS). The predicted phase diagram has upper critical solution temperature (UCST) behavior with a critical temperature of 143 degreesC for the molecular weights used in this study. OM was used to locate cloud points and the results are consistent with the predicted phase diagram. Since iPP melts above the critical point, care was taken to distinguish phase separation front iPP crystallization by studying the kinetics of iPP crystallization, and the iPP crystallization was discerned from dewetting. In PE-rich blends, the iPP crystallization was suppressed and no dewetting was observed. (C) 2004 Published by Elsevier Ltd.