The photochemistry of chloro-(ClBQ), dichloro-(2,5- and 2,6-Cl(2)BQ), and trichloro-1,4-benzoquinone (Cl(3)BQ) was studied in aqueous solution and/or in mixtures with acetonitrile. Final products are the corresponding hydroquinones (QH(2)s) and 2-hydroxy-1,4-benzoquinones (QOHs). Three transients were detected by UV-vis absorption spectroscopy. The triplet state appears within the 20 ns 248 nm pulse and is converted within 0.1-1 mu s into a photohydrate (HIaq). HIaq which is spectroscopically and kinetically separated from the triplet state decays within 5 ms, whereas the anion of the hydroxyquinone (QO(-)) grows in at ca. 500 nm in the 0.1-1 s time range. The proton formation and decay kinetics within 0.1-10 mu s were observed by transient conductivity in the course of the reaction of the triplet state with water en route to HIaq at pH 4-9. Formation of QO(-) results in a permanent conductance. The efficient photoconversion of Cl(n)BQs at low concentrations (< 0.2 mM) into QH(2)s and HOQs is due to a redox reaction of Q with rearranged HIaq. The quantum yield of photoconversion at lambda(irr) = 254 nm is 0.8-1.2 for ClBQ or Cl(2)BQs in aqueous acetonitrile and smaller (0.4) for Cl(3)BQ. The yield of semiquinone radical ((center dot)QH/Q(center dot-)) of Cl(n)BQs (n = 1-4) in acetonitrile-water (1:1) is low (< 20%) at low substrate concentration but is significantly increased upon addition of an H-atom donor, for example, 2-propanol. Other mechanisms involving (center dot)QH/Q(center dot-) radicals, such as quenching of the triplet state at enhanced Cl(n)BQ concentrations and H-atom abstraction from an organic solvent in mixtures with water, have also to be considered.