Keto-enol equilibria in catechol have been studied using ab initio methods and first principle density functional theory. Six structural isomers of CHO were fully optimized in gas phase at HF and B3LYP levels of theory in combination with the 6-311++G** basis set. Self-Consistent Reaction Field Polarizable Continuum Model (SCRF-PCM) was used to investigate the effect of an aqueous solvent on the extent of tautomerisation at the two above mentioned model chemistries. In addition, gas phase electronic and Gibbs free energies for the six molecules were computed using the CBS-QB3 method, and the G3 approach to obtain highly accurate relative energies. It was found that the two dienol isomers are always lower in energy than the other species, and predominate. Resonance stabilization arising from aromaticity in these six-member cyclic systems is posited, and is understood on the basis of Huckel’s theory.