Thermodynamic parameters for the benzene oxide <=> oxepin system have been calculated at MP4(SDQ)/6-31+G**//HF/6-31G** level of theory. The calculated enthalpy for this valence tautomeric equilibrium differs from that reported by Vogel et al in 1967, but agree well with the value calculated by Kollman using the MINDO/3 method. Large deviations in the experimental tautomerization entropies lead to unreliable delta Gº values. The differences in delta Hº and delta Sº can be due to the lack of band shape analysis of the ¹H-NMR spectra. The effect of solvent polarity on the above equilibrium has been studied using the isodensity polarized continuum method (IPCM). Low polar solvents favor the oxepin formation whereas medium to high polar solvents leads to benzene oxide formation. The transition state for the tautomerization reaction has been fully characterized and the activation energies for the forward and reverse reaction are estimated to be ca. 9.5 and 11.0 kcal/mol,respectively. The solvent polarity exerts a reasonable effect decreasing the activation energies up to 4 kcal/mol.