To investigate the effect of alkane molecular structure on the volume expansion of CO2 + alkane systems, the solubility of CO2 in hexane, octane, decane, and cyclohexane and the volume of CO2 + alkanes at different temperature and pressure was measured with a PVT apparatus. The results imply that the dispersion state of CO2 molecules in the alkane phase under near critical or supercritical condition of CO2 plays a dominate role in increasing the volume of CO2 + alkane systems. The solubility of CO2 in the alkanes, the volume expansion of the CO2 + alkane systems, and the London force operating within alkane molecules are strongly influenced by molecular structure of the alkanes and pressure.