FTIR was used to investigate the effect of solvent and solvent density on the keto-enol tautomeric equilibrium in compressed and supercritical carbon dioxide as well as ethane. The effect of the solvent (CO2 and ethane) density on the tautomerism was found to be insignificant. These experimental results are in good agreement with modeling studies, which combine ab initio calculations of isolated acetylacetone with reactive Monte Carlo simulations of the CO2 and ethane solvation effects. (c) 2005 Elsevier B.V. All rights reserved.