Carbazole/cyanobenzene photocatalysts promote the direct isotopic carboxylate exchange of C(sp3) acids with labeled CO2. Substrates that are not compatible with transition-metal-catalyzed degradation-reconstruction approaches or prone to thermally induced reversible decarboxylation undergo isotopic incorporation at room temperature in short reaction times. The radiolabeling of drug molecules and precursors with [11C]CO2 is demonstrated.