A thermodynamic investigation of the Claisen rearrangement of chorismate(2-)(aq) to prephenate(2-)(aq) has been performed using microcalorimetry and high-performance liquid chromatography. The study used a well-characterized monofunctional chorismate mutase from Bacillus subtilis that was devoid of prephenate dehydrogenase and prephenate dehydratase activities. The calorimetric measurements led to a standard molar enthalpy change Delta(r)H(m)(0) = -(55.4 +/- 2.3) kJ mol(-1) at 298.15 M for this reaction. An estimated value of the standard molar entropy change Delta(r)S(m)(0) = 3 J K-1 mol(-1) for the above reaction was used together with the experimental value of Delta(r)H(m)(0) to obtain a standard molar Gibbs free energy change Delta(r)G(m)(0) approximate to -56 kJ mol(-1) and an equilibrium constant K approximate to 7 x 10(9) for the conversion of chorismate(2-)(aq) to prephenate(2-)(aq) at 298.15 K. Thus, for all practical purposes, this reaction can be considered to be "irreversible". Quantum mechanics (Gaussian 94 with a B3LYP functional and a 6-31G(*) basis set) was used to calculate values of absolute and relative energies for the chorismic acid and prephenic acid species having charge numbers 0 and -2 both in the gas phase and in-aqueous solution. The structures of prephenic acid and its dianion were also obtained along with values of thermodynamic reaction quantities. The effects of water solvation and solvent polarization were accounted for by using a self-consistent isodensity polarized continuum model (SCI-PCM). The calculated value of Delta(r)H(m)(0) for the conversion of chorismate(2-)(aq) to prephenate(2-)(aq) at 298.15 K was -46.4 kJ mol(-1). This very good agreement between theory and experiment suggests that the energetics of this Claisen rearrangement an well understood and that the SCI-PCM method is capable of adequately describing the increased solvent-solute interaction of prephenate(2-) relative to chorismate(2-).