Abstract. Organic carbon (OC) depleted in 13 C is a widely used tracer for terrestrial organic matter (OM) in aquatic sys- tems. Photochemical reactions can, however, change δ 13 C of dissolved organic carbon (DOC) when chromophoric, aromatic-rich terrestrial OC is selectively mineralized. We assessed the robustness of the δ 13 C signature of DOC ( δ 13 C DOC ) as a tracer for terrestrial OM by estimating its change during the photobleaching of chromophoric DOM (CDOM) from 10 large rivers. These rivers cumulatively ac- count for approximately one-third of the world’s freshwater discharge to the global ocean. Photobleaching of CDOM by simulated solar radiation was associated with the photochem- ical mineralization of 16 to 43 % of the DOC and, by prefer- entially removing compounds depleted in 13 C, caused a 1 to 2.9 ‰ enrichment in δ 13 C in the residual DOC. Such solar- radiation-induced photochemical isotopic shift could bias the calculations of terrestrial OM discharge in coastal oceans to- wards the marine end-member. Shifts in terrestrial δ 13 C DOC should be taken into account when constraining the terres- trial end-member in global calculation of terrestrially derived DOM in the world ocean.