The potential energy surface of the ground state and lowest lying singlet of o-hydroxyacetophenone was calculated at the ab initio SCF and CIS levels, respectively, using a split-valence basis set to which was added polarization functions on atoms involved in the intramolecular H-bond. Whereas these potentials each contain one minimum corresponding to the normal form and a second well for the proton-transferred tautomer, only one minimum appears in these potentials after correlation is added. SO - S1 excitation hence induces the tautomerization associated with intramolecular proton transfer from the hydroxyl to the carbonyl oxygen. The excited state proton transfer may involve a nonradiative transition from the Sl(nn*) state to Sl(nn*). The vibrational spectra of the two tautomers have certain strong differences. Anharmonic adiabatic treatment of the OH* 9 segment of the molecule leads to improved reproduction of experimentally observed v ( 0 9) and v(0H) frequencies.