The structures and harmonic force fields of p-benzoquinone in various oxidation and protonation states have been calculated by means of density functional methods. In general, good agreement between experimental and theoretical vibrational frequencies in the 1400-1700-cm(-1) frequency region has been found. Occasionally, the applied BP86 density functional method combined with the 6-31G** basis set results in poor agreement with experimental data. The inaccurate prediction of a ring-bending mode and of CH out-of-plane vibrations in benzene, p-benzoquinone, and p-hydroquinone can most likely be identified as a systematic error of the applied method. Such modes which are poorly described with this quantum chemical method can occasionally be better reproduced when a smaller basis set of 3-21G quality or a hybrid Hartree-Fock/density functional method is applied.