The OH stretching vibrations of phenol (Ph), phenol cation ([Ph](+)), phenol-water complex (Ph-H2O) and phenol-water cationic complex ([Ph-H2O](+)) are investigated theoretically in terms of ab initio potential energy surfaces, vibrational wavefunctions, and infrared absorption intensities. It was found that the anharmonic coupling between the hydrogen-bonded OH stretching and bending vibrations in [Ph-H2O](+) is much stronger than those in Ph, [Ph](+) and Ph-H2O, resulting in vibrational wavefunction mixing and infrared intensity borrowing. (C) 2003 Elsevier Science B.V. All rights reserved.