One-electron reductants (OER) photoproduced by chromophoric dissolved organic matter (CDOM) have been shown to be likely precursors for the formation of superoxide and subsequently hydrogen peroxide. An improved method that employs a nitroxide radical probe (3AP) has been developed and utilized to determine the photoproduction rates of OER from a diverse set of CDOM samples. 3AP reacts with OER to produce the hydroxylamine, which is then derivatized with fluorescamine and quantified spectrofluorometrically. Although less sensitive than traditional methods for measuring R O2•− , measuring R H provides a simpler and faster method of estimating R O2•− and is amenable to continuous measurement via flow injection analysis. Production rates of OER (R H ), superoxide (R O2•− ), and hydrogen peroxide (R H2O2 ) have a similar wavelength dependence, indicating a common origin. If all the OER react with molecular oxygen to produce superoxide, then the simplest mechanism predicts that R H /R H2O2 and R O2•− /R H2O2 should be equal to 2. However, our measurements reveal R H /R H2O2 values as high as 16 (5.7−16), consistent with prior results, and R O2•− /R H2O2 values as high as 8 (5.4−8.2). These results indicate that a substantial fraction of superoxide (65−88%) is not undergoing dismutation. A reasonable oxidative sink for superoxide is reaction with photoproduced phenoxy radicals within CDOM.