Exchange reactions of hexafluoroacetylacetonate (hfacac) in UO2(hfacaC)(2)DMSO (DMSO = dimethyl sulfoxide) in o-C6D4Cl2 and supercritical CO2 (SC-CO2) have been studied using the NMR line-broadening method to compare reactivity in a nonaclueous solvent with that in SC-CO2. It was found that the exchange rates of hfacac in both systems are dependent on the concentration of the enol isomer ([Henol]) of hexafluoroacetylacetone and become slow with an increase in the concentration of free DMSO ([DMSO]). The exchange reaction between free and coordinated DMSO in UO2(hfacaC)(2)DMSO has been also examined in o-C6D4Cl2 and SC-CO2. As a result, the exchange rate of DIMSO was found to depend on [DIMSO]. From these results, the hfacac exchange reactions in UO2(hfacac)(2)DMSO in o-C6D4Cl2 and SC-CO2 were proposed to proceed through the mechanism that the ringopening for one of two coordinated hfacac in U02(hfacac)2DMSO is the rate-determining step, and the resulting vacant site is coordinated by the incoming Henol, followed by the proton transfer from Henol to hfacac and the ring closure of unidentate hfacac. The rate constants at 60 degrees C and the activation parameters (Delta H-double dagger and Delta S-double dagger) for the ring-opening path are 35.8 +/- 3.2 s(-1), 57.8 +/- 2.7 KJ.mol(-1), and -42.9 +/- 7.7 J.mol(-1).K-1 for the o-C6D4Cl2 system, and 518 +/- 50 s(-1), 18.9 +/- 1.8 KJ.mol(-1), and -138 +/- 5 J-mol(-1).K-1 for the SC-CO2 system, respectively. Differences in kinetic parameters between SC-CO2 and o-C6D4Cl2 Systems were proposed to be attributed to the solute-solvent interactions such as Lewis acid-Lewis base interactions and hydrogen bondings between SC-CO2 and beta-diketones.