Abstract The synthesis of isotopomers of phenylpyruvic acid, PPA, selectively labeled with hydrogen isotopes in the 3-position of the side-chain is reported. Three deuterium or tritium labeled isotopomers of L-phenylalanine, L-Phe, ie [(3S)-2 H]-L-,[(3S)-3 H]-L-, and doubly labeled [(3S)-2 H/3 H]-L-Phe were synthesized using the enzyme phenylalanine ammonia lyase (EC 4.3. 1.5). In the second step these isotopomers of L-Phe were converted into [( ...