The keto-enol tautomeric equilibrium of 5,5-dimethyl-1,3-cyclohexanedione (dimedone) was investigated using UV-Vis spectroscopy in various solvents, including liquid acetonitrile (CH3CN), supercritical carbon dioxide (SC CO2) and SC CO2-CH3CN. The content of the enol tautomer in liquid CH3CN is much higher than that in SC CO2. With the addition of a small amount of cosolvent CH3CN into SC CO2, the enol content increases with respect to that in pure SC CO2. The comparison of thermodynamic functions for the tautomerism was made in these solvents. It was found that the change of standard internal energy (Delta U-0) in SC CO2-CH3CN is close to that in liquid CH3CN and remarkably different from that in pure CO2. On the other hand, the maximum decrease in the change of the standard entropy (Delta S-0) was observed in SC CO2-CH3CN. It indicates that the cosolvent preferentially aggregates about the two tautomers. The Delta S-0 becomes the dominant thermodynamic driving force of the tautomerization in the cosolvent-modified SC CO2.