We present combined quantum mechanical and free energy (QM-FE) as well as molecular dynamics (MD) calculations to investigate the methyl transfer reaction catalyzed by the enzyme catechol 0-methyltransferase (COMT). The calculated transition state free energy, Delta G(double dagger), of 24.5 kcal/mol for the enzymatic reaction is in reasonable agreement with experiment (18 kcal/mol). and the level of agreement improves when the substrates are allowed to slightly deviate from their molecular mechanical energy minimized interpolated geometries. The calculated Delta G(double dagger) for the reaction in water is 5 kcal/mol higher than that found in the enzyme when cratic free energy terms are not considered, and this difference increases to 14-18 kcal/mol when they are included, in very good agreement with the estimated rate enhancement due to enzyme catalysis of 10(11) (corresponding to Delta Delta G(double dagger) approximate to 15 kcal/mol). In contrast to trypsin, studied earlier by QM-FE methods, the calculated gas phase Delta G(double dagger) is significantly lower than observed in the enzyme or in solution; thus, in COMT the enzyme must avoid increasing the transition state barrier as much as is observed in solution, in addition to these free energy calculations, we describe MD simulations on various monosubstituted catechols and we use such calculations to rationalize the significant regioselectivity for attack at the meta rather than para OH group relative to the substituent.