The photolyses of phosphate-buffered (pH 7) air- and nitrogen-saturated solutions containing the water-soluble quinones, 1,4-benzoquinone (BQ), 2-methyl-1,4-benzoquinone (MBQ), sodium 1,4-naphthoquinone-2-sulfonate (NQ2S), 9,10-anthraquinone-2-sulfonate (AQ2S) or 9,10-anthraquinone-1,5-disulfonate (AQDS), and the spin trap 5,5-dimethylpyrroline-1-oxide (DMPO) produce a DMPO-OH adduct. Electron paramagnetic resonance spectroscopy of the photolyzed samples in 17O-enriched water demonstrates that this adduct derives almost exclusively from water. With the exception of BQ, quantum yields for the formation of DMPO-OH are larger in air than in nitrogen-saturated samples, thus supporting the idea of the formation of air-oxidized intermediates that enhance the DMPO hydroxylation reaction rate. Evidence has been obtained which suggests that BQ and MBQ, but not AQDS, are able to photooxidize water, with the consequent production of the free OH radical.