The cycling of carbon on the Arctic shelves, including outgassing of CO2 to the atmosphere, is not clearly understood. Degradation of terrestrial organic carbon (OCter) has recently been shown to be pronounced over the East Siberian Arctic Shelf (ESAS), i.e. the Laptev and East Siberian Seas, producing dissolved inorganic carbon (DIC). To further explore the processes affecting DIC, an extensive suite of shelf water samples were collected during the summer of 2008, and assessed for the stable carbon isotopic composition of DIC (δ13CDIC). The δ13CDIC values varied between −7.2‰ to +1.6‰ and strongly deviated from the compositions expected from only mixing between river water and seawater. Model calculations suggest that the major processes causing these deviations from conservative mixing were addition of (DIC) by degradation of OCter, removal of DIC during primary production, and outgassing of CO2. All waters below the halocline in the ESAS had δ13CDIC values that appear to reflect mixing of river water and seawater combined with additions of on average 70 ± 20 μM of DIC, originating from degradation of OCter in the coastal water column. This is of the same magnitude as the recently reported deficits of DOCter and POCter for the same waters. The surface waters in the East Siberian Sea had higher δ13CDIC values and lower DIC concentrations than expected from conservative mixing, consistent with additions of DIC from degradation of OCter and outgassing of CO2. The outgassing of CO2 was equal to loss of 123 ± 50 μM DIC. Depleted δ13CPOC values of −29‰ to −32‰ in the mid to outer shelf regions are consistent with POC from phytoplankton production. The low δ13CPOC values are likely due to low δ13CDIC of precursor DIC, which is due to degradation of OCter, rather than reflecting terrestrial input compositions. Overall, the δ13CDIC values confirm recent suggestions of substantial degradation of OCter over the ESAS, and further show that a large part of the CO2 produced from degradation has been outgassed to the atmosphere.