Sorption and volume dilation isotherms of semicrystalline poly(4-methyl-1-pentene) (PMP) were measured using CO2 and C3H8 as penetrants, which have sieving diameters of 3.3 and 4.3 Angstrom, respectively. On the other hand, the PMP crystal has a void width of approximately 4 Angstrom, as estimated by X-ray diffraction, so it was anticipated that CO2 would be able to sorb into the PMP crystal while C3H8 would not. The data show that C3H8 has a constant partial molar volume of approximately 87 cc/mol, just above the value reported in other rubbery polymers, and are consistent with the hypothesis that the C3H8 molecules are too large to sorb into the PMP crystals. The partial molar volume of CO2 was found to be 39 cc/mol for CO2 weight fractions of up to 0.03. Since the typical partial molar volume of CO2 in rubbery materials is 46 cc/mol, the lower values in this study were attributed to CO2 sorption into crystalline regions of the polymer, which provided no dilation. Application of a two-phase model using the assumption of Henry’s law sorption showed that apparently all C3H8 sorption was occurring in the amorphous region but approximately 16% of CO2 sorption occurred in the crystalline regions.