When CO2 dissolves into a polypropylene (PP), its crystallization kinetics changes. These changes were studied, in the expectation that the information would reflect on the behavior of other semicrystalline polyolefins. The isothermal crystallization rate of the PP-CO2 solutions was measured using a high-pressure differential scanning calorimeter (DSC), which performed calorimetric measurements while keeping the polymer in contact with pressurized CO2. Although the measured crystallization rate followed the Avrami equation, the value of the crystallization kinetic constant was different from that measured for PP crystallized in air under atmospheric pressure. The dissolved CO2 decreased the overall crystallization rate of PP within the nucleation dominated temperature region. This suggests that the dissolved CO2 decreases the melting and the glass transition temperatures and prevents formation of critical size nuclei.