In acidic aqueous solutions UO22+ serves as a photocatalyst (lambda(irr) greater than or equal to 425 nm) for the oxidation of benzene by H2O2 Under conditions where 50% of the excited stare *UO22+ is quenched by H2O2 (k = 5.4 x 10(6) M(-1) s(-1)) and 50% by benzene (k = 2.9 x 10(8) M(-1) s(-1)), the quantum yield for the formation of phenol is 0.70. The yield does not change when benzene is replaced by benzene-d(6), but decreases by a factor of similar to 4 upon the change of solvent from H2O to D2O. Photocatalytic oxidation of toluene by UO22+/H2O2 produces PhCHO, PhCH(2)OH, and a mixture of cresols with a total quantum yield of 0.28 under conditions where 50% of *UO22+ is quenched by H2O2. The quenching of *UO22+ by benzene and substituted benzenes takes place with k > 10(8) M(-1) s(-1). The system UO22+/t-BuOOH/C6H6/hv does not result in the oxidation of benzene, but instead yields methane and ethane.