We report studies of supersonically cooled o-aminobenzoic acid using two-color resonantly enhanced multiphoton ionization and zero kinetic energy photoelectron spectroscopy. Vibrational modes of the first electronically excited state (S-1) of the neutral species and those of the ground state cation (D-0) have been assigned, and the ionization threshold has been determined. The results are analyzed in comparison with the other two isomers of aminobenzoic acid, mainly in terms of electron back donation of the two substituents in both the S-1 and the D-0 states. The change in the intramolecular hydrogen bond upon electronic excitation and ionization will also be discussed. (C) 2004 Elsevier B.V. All rights reserved.