Inelastic neutron scattering and solid state quantum chemistry calculations are applied to investigate the lattice vibrations in crystalline potassium hydrogen phthalate containing very short intermolecular OH⋯O hydrogen bond (2.529Å). The inelastic neutron scattering spectrum is interpreted in the energy range 30–3500 cm−1 using the density function theory approach within the harmonic approximation. The one-dimensional potential of the proton moving along the short OH⋯O bond is mapped and a considerable contribution of anharmonicity (about 24%) to the antisymmetric OHO stretching vibration is calculated.