An integrated catalytic decarboxylation/carboxylation for accessing isotopically labeled carboxylic acids with ¹³CO₂ or ¹⁴CO₂ is described. The method shows a wide scope under mild conditions, even in the context of late-stage functionalization, and does not require stoichiometric organometallics, thus complementing existing carbon-labeling techniques en route to carboxylic acids.