Structure and harmonic force fields of methoxy-substituted p-benzoquinones have been calculated applying density functional methods. Structural parameters, harmonic force constants, and vibrational frequencies have been shown to depend on the orientation of the methoxy substituents, Stable conformations of the methoxy groups, as predicted by semiempirical or ab initio Hartree-Fock methods, differ qualitatively from those predicted by methods which are considering correlation effects. The agreement of the calculated C=C and C=O modes with experimental data is generally very satisfactory. Due to additional substituents such as methyl or allyl groups, the energetic sequence of C=C and C=O modes is not found to be altered. Calculated frequencies are only slightly affected due to such substituents whereas intensities, mode decompositions, and isotopic shifts are considerably influenced.