We experimentally determined the density and mole fraction of CO2 (X-CO2) for CO2-alcohol (methanol, ethanol, propanol, butanol, isopropyl alcohol, and tert-butyl alcohol) mixtures and performed molecular dynamics (MD) simulations to study the mechanisms of volume expansion at 40 degrees C. The volume as calculated by vapor-liquid equilibrium (VLE) data increased with decreasing alkyl chain length, although there was no effect of branched alkyl groups. Analysis of the hydrogen bond network showed that the average number of hydrogen bonds per alcohol molecule decreased with increasing branched methyl groups. At pure alcohol condition, large size hydrogen bond networks were made. With further addition of CO2 molecules, it became difficult to contain the large hydrogen bond networks. Furthermore, the hydrogen bond networks changed to a cyclic pentamer or tetramer, and volume expansion occurred.