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The density and volumetric behavior of three typical n -alkanes (hexane, octane, and decane) influenced by different mole fractions of CO 2 injected in them at temperatures from 303 to 363 K and pressures from 3.8 to 8.67 MPa were investigated by performing molecular dynamics simulations. It is shown that the mass density first increases and then decreases with increasing CO 2 mole fraction. Correspondingly, the system volume only slightly swells at low CO 2 contents while suddenly expanding when the CO 2 mole fraction exceeds a value of ∼60%. The calculations of structural properties and interaction energies indicate that at low CO 2 mole fractions, there are a few CO 2 molecules existing in the gap of alkane molecules, resulting in poor compressibility, while at higher CO 2 concentrations, the CO 2 molecules begin to separate from the CO 2 -saturated alkane phase and form a gas phase, leading to higher compressibility. Therefore, at high CO 2 mole fractions, the system density and volume can more easily be changed by temperature and pressure than that at low CO 2 mole fractions. In addition, since it is harder for alkanes with longer chains to separate from each other, the volume swelling decreases and the density increases with increasing carbon number of n -alkane chains. Finally, we found that the increase in CO 2 mole fraction, temperature, and the decrease in alkane chain length would promote the diffusion of both CO 2 and alkane molecules. However, the influence of pressure on molecular diffusion is very limited except when P = 8.67 MPa and T = 333 K, where CO 2 is in the supercritical state. This work is helpful for understanding the density and volumetric behavior of n -alkane/CO 2 mixtures at a molecular level and provides useful information for guiding carbon sequestration and CO 2 -enhanced oil recovery., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)