In the present study, the diffusion coefficients (D) of vitamin K-3 (2-methyl-1,4-naphthalenedione) in carbon dioxide-expanded liquid methanol were measured by the Taylor dispersion method close to the gas liquid saturation line at 313.2 K over nearly the entire range of carbon dioxide mole fractions (xCO(2)) from zero (pure methanol) to 0.89. Although the D values simply increased with increasing CO2 mole fractions when increasing the pressure, similar to the behavior of the expansion coefficients, there were two unique regions with respect to solvent density: from xCO(2) = 0 to ca. 0.44 (corresponding to the almost maximum solvent density at 313.2 K) and from this density to the critical point of the mixture. In the former region, the D values increased with increasing pressure (while the solvent density also increased) and in the latter region D increased with decreasing solvent density, while the density also decreased with increasing pressure or xCO(2). The resulting D values were less than those predicted by the hydrodynamic equation when using two constants determined for vitamin K-3 in supercritical CO2 and in organic solvents. The relative differences of diffusion coefficients, representing the deviations from the hydrodynamic equation, were almost proportional to the excess viscosity values. (C) 2015 Elsevier B.V. All rights reserved.