Detailed kinetic studies were performed on the reaction of [Ru-II(terpy)(bpy)H](+) (terpy = 2,2',6',2 ''-terpyridine; bpy = 2,2'-bipyridirie) with CO2 in conventional solvents (water, methanol, and ethanol) and in the ionic liquid [emim][NTf2] ([emim] = 1-ethyl-3-methyl-imidazolium; [NTf2] = bistrifluoromethylsulfonylamide). Second-order rate constants and activation parameters (Delta H double dagger, Delta S double dagger, and Delta V double dagger) were determined for the reaction in all solvents. The second-order rate constants correlate with the acceptor number of the solvent, whereas the activation parameters support the associative nature of the reaction. The results in water, especially the activation entropy (+14 +/- 2 J K-1 mol(-1)) and activation volume (-5.9 +/- 0.6 cm(3) mol(-1)) differ significantly from those found for the other solvents.