The OH stretching vibrations of phenol (Ph), phenol cation ([Ph] + ), phenol-water complex (Ph-H 2 O) and phenol-water cationic complex ([Ph-H 2 O] + ) 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-H 2 O] + is much stronger than those in Ph, [Ph] + and Ph-H 2 O, resulting in vibrational wavefunction mixing and infrared intensity borrowing.