The excess molar volumes of 10-40 mol % methanol/CO2 mixtures at 26 degrees C as a function of pressure were determined. The excess molar volumes varied with composition and pressure, Significant interaction between CO2 and methanol was noted from the observed excess molar volumes. For all conditions studied the excess volumes were negative. The largest negative values were observed at the lowest pressures studied (approximate to 80 atm). To better characterize the interaction and its effect on analyte solubility, the partial molar volume of napthalene at infinite dilution in liquid 10 and 40 mol % methanol/CO2 mixtures was determined, Large negative values of the partial molar volume of naphthalene at infinite dilution were observed for the 10 mol % methanol/CO2 mixtures at low pressures (approximate to 80 atm). The variation of the partial molar volume at infinite dilution with pressure correlated well with isothermal compressibility of the methanol/CO2 mixtures.