Isotactic polypropylene (PP) has been blended with various types of polyethylene, high density (HDPE), low density (LDPE), linear low density (LLDPE), very low density (VLDPE) and ultra low density (ULDPE). Each blend contained 20% by mass PP. The blends were cooled from the melt to temperatures where PP could crystallise, but not the polyethylene. When the two polymers were immiscible, or immiscible at the crystallisation temperature where liquid-liquid phase separation occurred on cooling, then the two phases crystallised independently. Under these conditions the crystallisation rate (half-time) for PP was very similar to that of purl polypropylene. When the polymers were miscible, crystallisation of PP took place from a solution in the molten polyethylene. Under these conditions the crystallisation rate of PP was greatly decreased since it was in dilute solution. The significant change in rate of crystallisation of PP was a detection of miscibility. After PP had crystallised the blend was cooled to ambient temperature and the polyethylene quickly crystallised in the intervening spaces. When PP crystallised from a homogeneous solution, which was the case with only LLDPE, broad diffuse spherulites formed and PP became a continuous phase. Crystallisation under these conditions took 5-10 h and a unique co-continuous structure resulted even though PP was only present at 20%. PP was found to be immiscible in HDPE, LDPE and VLDPE. A combination of DSC and hot stage polarised optical microscopy was used to study the crystallisation of the blends.