We measured the density and liquid-phase CO2 mole fraction of CO2-expanded ketones (acetone, methyl ethyl ketone, methyl propyl ketone, methyl butyl ketone, methyl i-propyl ketone, and t-methyl butyl ketone) at 40 degrees C and evaluated the expansion coefficient on the basis of the experimental results. Volume expansion was slow in the region of a low carbon dioxide mole fraction (<0.7) but rapid in the region of a high carbon dioxide mole fraction (>0.7). The dependence of the expansion coefficient on the carbon dioxide mole fraction could be attributed to the number of molecules in a certain volume. These phenomena were inferred from the partial molar volume estimated using the Peng-Robinson equation of state (PR-EOS). (C) 2010 Elsevier B.V. All rights reserved.