Experimental free energies of tautomerization, Delta G(T), were used to benchmark the gas-phase predictions of 17 different quantum mechanical methods and eight basis sets for seven keto enol tautomer pairs dominated by their enolic form. The G4 method and M06/6-31+G(d,p) yielded the most accurate results, with mean absolute errors (MAE's) of 0.95 and 0.71 kcal/mol, respectively. Using these two theory levels, the solution-phase Delta G(T) values for 23 unique tautomer pairs composed of aliphatic ketones, beta-dicarbonyls, and heterocycles were computed in multiple protic and aprotic solvents. The continuum solvation models, namely, polarizable continuum model (PCM), polarizable conductor calculation model (CPCM), and universal solvation model (SMD), gave relatively similar MAE's of similar to 1.6-1.7 kcal/mol for G4 and 4.9-2.0 kcal/mol with M06/6-31+G(d,p). Partitioning the tautomer pairs into their respective molecular types, that is, aliphatic ketones, beta-dicarbonyls, and heterocycles, and separating out the aqueous versus nonaqueous results finds G4/PCM utilizing the UAO cavity to be the overall most accurate combination. Free t energies of activation, Delta G(double dagger), or the base-catalyzed keto enol interconversion of 2-nitrocydohexanone were also computed using six bases and five solvents. The M06/6-31+G(d,p) reproduced the AGt with MAE's of 1.5 and 1.8 kcal/mol using CPCM and SMD, respectively, for all combinations of base and solvent. That specific enolization was previously proposed to proceed -via a concerted mechanism in less polar solvents but shift to a stepwise mechanism in more polar solvents. However, the current calculations suggest that the stepwise mechanism operates in all solvents.