Abstract The torsional potential energy curve of the hydroxyl group of hydroquinone and tetramethyl-hydroquinone cation radicals were explored with various ab initio methods. The minimum and the torsional transition state geometries and energies were computed by using high accuracy density functional methods yielding the rotation barrier height and the energy difference between the cis- and trans-isomers. The obtained minimum energy geometry for the hydroquinone cation radical indicates that the CO bond has shortened when compared to the neutral species. We attribute this to the increased double-bond character of this bond. The energy minima were located for methyl-hydroquinone, 2,3-dimethyl-hydroquinone, 2,5-dimethyl-hydroquinone, 2,6-dimethylhydroquinone and trimethyl-hydroquinone cation radicals. By assuming the Boltzman distribution among the energy levels of the cis- and trans-isomers of these compounds the ratio of expected isomers at 230K was obtained.