Effective quantum yields for the production of OH radicals from iron(III) hydroxo species in aqueous solution were measured in the wavelength range 280-370 nm at pH 2 and 3 under aerated conditions and at pH 3 in argon-saturated solutions of ferric perchlorate. 2-Propanol was used as radical scavenger; the yield of product acetone was measured. Spectra of ferric perchlorate solutions were taken and evaluated to determine absorption coefficients at various pH values and to calculate K-4, the first hydrolysis constant of Fe(III), at 46 wavelengths in an attempt to obtain evidence for the presence of Fe(OH)(2)(+) in addition to FeOH2+ in the spectra. Whereas the influence of the dimer Fe-2(OH)(2)(4+) was clearly evident, no indication for Fe(OH)(2)(+) was found. Values obtained for the first hydrolysis constant K-4, corrected to 25 degrees C, were (2.76 +/- 0.19) x 10(-3) M and (3.63 +/- 0.15) x 10(-3) M at ionic strengths of 0.1 and 0.05, respectively, in excellent agreement with literature values. These data served to derive absolute OH quantum yields for FeOH2+(H2O)(5), which were found to rise from 0.07 at 370 nm to 0.31 at 280 nm.,At wavelengths below 300 nm the OH production from Fe3+(H2O)(6) contributed markedly with an estimated quantum yield of approximate to 0.05. Quantum yields of SO4- from the FeSO4+ complex were determined in solutions of ferric perchlorate, to which of sodium sulfate was added at pH 2. Benzene was used as scavenger, and sufficient 2-methyl-2-propanol was added to quench OH radicals resulting from iron(III) hydroxo species. Absolute SO4- quantum yields were found to rise from 1.6 x 10(-3) at 350 nm toward 7.9 x 10(-3) at 280 nm.