The pressure and cooling rate dependence of the phase diagram of isotactic polypropylene (iPP) with the nucleating agent l,3:2,4-bis(3,4- dimethylbenzylidene)-sorbitol (DMDBS) is investigated. A custom designed dilatometer is used to measure the specific volume of the blends for a wide range of cooling rates and elevated pressures. The crystallization line in the phase diagram shifts to higher temperatures with increase in the pressure and decrease in the cooling rate, independent of the concentration. The influence of cooling rate and pressure is related to the final morphology determined from X-ray diffraction. Dilatometry can be used as an interesting alternative to extract information on the phase behavior and crystallinity, for conditions hard or not at all obtainable with standard techniques like DSC or SALS.