The vibrational bands experimentally observed for aqueous p-benzosemiquinone radical anion are analyzed by use of electronic structure calculations. To clarify the effects of solvation, calculations on the isolated semiquinone anion are compared to those on supermolecule clusters having two to four water molecules strongly hydrogen-bonded at the semiquinone oxygen atoms. The results allow satisfactory assignment of all experimentally known aqueous vibrational bands, some being assigned here for the first time. The clusters considered give reasonable accounts both of the observed frequencies in water and of the frequency shifts between acetonitrile and water that arise from hydrogen bonding.