This article discusses the specific volume-measurement methods for semicrystalline polymers needed in order to obtain reliable data. Particularly, the effect of the cooling rate is analyzed, taking into account the thermal gradient in a cylindrical sample. Experimental results for a polypropylene form the basis for the study. In a first step our thermal model was validated by comparing the calculated results with the experimental ones for a temperature range higher than the crystallization zone with different cooling rates and by analyzing the stabilization time of the measured specific volume after cessation of the cooling. Secondly, specific-volume evolutions from 220 degreesC to 50 degreesC for different cooling rates and different pressures were analyzed, revealing that when the data are corrected to eliminate the thermal gradient effect, the transition zone is much narrower than the experimental one. Moreover, the effect of the pressure and the cooling rate on the relative crystallinity function-that is, on the crystallization kinetics-can be more accurately evaluated.