Summary: We used a custom designed dilatometer to measure quantitatively the time evolution of the specific volume of (semi-crystalline) polymers for an unusual combined wide range of cooling rates and elevated pressures, covering processing conditions. For an isotactic poly(propylene), applying the Schneider rate equations for quiescent crystallization, experimental results are compared to numerical predictions. Average cooling rates imposed during crystallization of the material vary from 0.1 to 35 °C · s-1 while pressures range from 20 to 60 MPa. The results show the well-known profound influence of pressure and cooling rate on the specific volume. An increasing cooling rate shifts the crystallization temperature Tc towards lower temperatures, increases the final specific volume, and the transition due to crystallization is more gradual and widespread. For the highest cooling rate applied, the shift in Tc is as much as 30 °C, while the final specific volume increases up to 1.4%. Increasing pressure has an opposite effect on the shift in Tc, while the final specific volume, after pressure release, also increases. Finally, a comparison of numerical predictions with experimental data shows that the crystallization temperature Tc is consistently predicted too low, and that the predictions at high cooling rate are sensitive to (small) variations in model parameters.