Monte Carlo simulations have been used to determine changes in free energies of solvation for the rearrangement of chorismate to prephenate in water and methanol. Structures and partial charges from the ab initio RHF/6-31G* calculations of Wiest and Houk were used for the pseudodiequatorial and pseudodiaxial conformers of chorismate and for the transition structure. Free energy perturbation calculations yielded the differences in free energies of solvation for the three structures. The calculations reproduce the observed 100-fold rate increase in water over methanol. The origin of the rate difference is traced solely to an enhanced population of the pseudodiaxial conformer in water, which arises largely from a unique water molecule acting as a double hydrogen bond donor to the C4 hydroxyl group and the side-chain carboxylate. A Monte Carlo simulation was also carried out for the transition structure bound to E. coli chorismate mutase in order to characterize the key interactions in the active site. Consistent with earlier computational results for the Claisen rearrangement of allyl vinyl ether and inferences from crystal structures, the Monte Carlo simulations reveal two hydrogen bonds to the enolic oxygen in the transition structure for both the uncatalyzed reaction in water and the enzyme-catalyzed rearrangement.