Structures and harmonic force fields of the methyl-substituted p-benzoquinones 2-methyl-1,4-benzoquinone, 2,3-dimethyl-1,4-benzoquinone, 2,6-dimethyl-1,4-benzoquinone, and 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone, DQ) have been calculated by using density functional methods. The calculations can successfully predict experimentally detected frequencies in the 1500-1700 cm(-1) region of the spectrum in all four molecules. Occasionally, the mode assignment predicted by the calculation differs from previously postulated assignments. In particular in the case of DQ the calculations can successfully predict (i) the downshift of the most intensive carbonyl mode from 1666 cm(-1) in p-benzoquinone to around 1640 cm(-1) in DQ due to a reduction of the force constant of the carbonyl bond and (ii) the appearence of two isotopic shifts of similar size upon O-18 labeling of both carbonyl groups as it has been found experimentally in vitro and in photosynthetic reaction centers.