Significant concentrations of nitrite NO 2 -were produced from the photodegradation of humic substances (HS) isolated from a variety of natural waters in coastal North Carolina. Nitrite concentrations were 40-118% higher after light exposure relative to initial levels, while no statistical differences were observed in dark controls before and after irradiation. The amount of nitrite produced upon irradiation was positively correlated to the concentration of HS added (r ) 0.97, p < 0.001) and to the length of irradiation. The average production rate, normalized to both humic substance concentration and time of light exposure, for all HS studied ranged from 9.7 to 17 [nM (W-h) -1 m -2 mg -1 ] × 10 5 . This translates into an average yearly nitrite production of 2 × 10 6 mol yr -1 in the top one meter of Onslow Bay. When natural waters were irradiated, changes in nitrite were influenced by both initial nitrite and humic substance concentrations. The rate of nitrite photochemical formation in typical coastal waters (4 nM h -1 ) was smaller than its direct photolysis rate (23 nM h -1 ) and smaller than reported rates of ammonium generation via DOM photodegradation (50 nM h -1 ) and of the same order of magnitude as NH 4 + incorporation during humification (1 nM h -1 ). The photochemical release of biologically available nitrite from biologically refractory humic substances has significant implications with respect to nitrite biogeochemistry and N cycling in natural waters and suggests HS bound N is more biogeochemically labile than previously thought.