The miscibility of isotactic polypropylene (PP) in blends with (a) polystyrene-block-poly(ethylene-stat-butylene)-block-polystyrene (SEBS) and processing oil, (b) poly(ethylene-co-propylene) (EPM), and (c) EPM and processing oil, has been studied using the method of melting-point depression. Near equilibrium melting temperatures were determined by applying the Hoffman-Weeks method to PP melting temperatures determined by DSC on samples crystallized isothermally for 20 h at 80, 100, 120 and 140 degrees C. A large melting point depression was observed for PP in blends with SEES and processing oil, suggesting that PP was soluble in the molten blends. On melting, the turbidity of the PP/SEBS/Oil blends increased, suggesting that the equilibrium melt contains two liquid phases. For PP in blends with EPM or EPM and processing oil, a melting-point depression was not observed. The results indicate why PP/SEBS/Oil blends, within a wide composition range, formed bicontinuous interpenetrating network structures on cooling from the melt, whereas all of the blends with EPM formed solid blends with one continuous phase and one dispersed phase.