Exposure of riverine waters to natural sunlight initiated alterations in stable carbon isotope ratios (Δ13C) of the associated dissolved organic carbon (DOC). Water samples were collected from two compositionally distinct coastal river systems in the southeastern United States-the Satilla River which has high DOC concentrations (10--35 mg/l), and the Altamaha River which has relatively low DOC concentrations (3--9 mg/l). Approximately 21--26% of the DOC was remineralized to DIC. The Δ13C of remineralized DIC was isotopically light (Δ13C=-28.8 to -32.2?) relative to the initial DOC (Δ13C=-27.4?), leaving a residual fraction of isotopically heavy (Δ13C=-25.8 to -26.6?) DOC. Photochemically-induced fractionation of 13C results from selective degradation of certain biochemical constituents including lignin phenols. These results are consistent with shifts in Δ13C of DOC observed during mixing of river and marine waters, suggesting that photochemically-induced alterations are a factor in determining these changes. ¿ 2001 American Geophysical Union