Site-selective deuteration of C–H bonds increases the lifetime and efficacy of drug molecules. Although effective methods to form C(sp2)–D bonds are known, processes for making C(sp3)–D bonds often have low site selectivity, require expensive and unrecoverable D2 gas, or use stoichiometric reagents. Here we report cost-efficient and site-selective reductive deuteration using a tandem electrochemical chemical palladium membrane reactor. This architecture mediates the chemical reaction of deuterium atoms (derived from reusable D2O in an electrochemical compartment) with alkynes, aldehydes...