Finally, we sought to incorporate an isotopic label using CO gas, which is particularly useful for studying reaction mechanisms and tracing the fate of reactants in complex mixtures. To achieve this, we leveraged the advantages of flow technology, including the high gas-liquid interfacial area and precise stoichiometric control over the gaseous component using mass flow controllers. Therefore, the use of flow technology allows for the precise control of the amount of CO gas required, which can be challenging to achieve in conventional batch reactors due to gas accumulation in the headspace. By applying our optimized carbonylation protocol with a lower amount of CO gas (5 equivalents), we successfully converted several representative C-H donors to their corresponding carbonylation products in yields comparable to those obtained with CO (1, 24-26, 30). This result demonstrates a practical application of our methodology for isotopic labeling of organic molecules. Methane reactions are performed with 10 mol% of FeCl3.6H2O as HAT photocatalyst, with a residence time of 8 h. f 20 mol% of catalyst loading applied for benzyl vinyl sulfone. g Reactions with CO (g) were performed with 5 equiv. of CO under otherwise unchanged reaction conditions.