A significant amount of carbon is transported to the ocean as dissolved organic carbon (DOC) in rivers. During transport, it can be transformed through microbial consumption and photochemical oxidation. In dark incubation experiments with water from the Tana River, Kenya, we examined the consumption of DOC through microbial decomposition and the associated change in its carbon stable isotope composition (delta C-13). In 15 of the 18 incubations, DOC concentrations decreased significantly by 10 to 60 %, with most of the decomposition taking place within the first 24-48 h. After 8 days, the remaining DOC was up to 3% more depleted in C-13 compared with the initial pool, and the change in delta C-13 correlated strongly with the fraction of DOC remaining. We hypothesize that the shift in delta C-13 is consistent with greater microbial lability of DOC originating from herbaceous C-4 vegetation than DOC derived from woody C-3 vegetation in the semi-arid lower Tana. The results complement earlier findings that the stable isotope concentration of riverine DOC does not necessarily reflect the proportion of C-3 and C-4-derived DOC in the catchment: besides spatial distribution patterns of different vegetation types, processing within the river can further influence the delta C-13 of riverine OC.