Simple modular reaction schemes that allow access to any isotopomer of protected serine and homoserine have been worked out. These systems could be simply converted into cysteine, selenocysteine, homocysteine, homoselenocysteine, the essential amino acid methionine, and selenomethionine by Mitsunobu chemistry. These sulfur‐ and selenium‐containing amino acids fulfil many essential roles in the living organism. In addition, homoserine could be converted in a few steps into optically active L‐vinylglycine. As well as the stable isotopes 13C, 15N, 17O, and 18O, the radioactive isotopes of sulfur, selenium and carbon can also be easily introduced in a site‐directed fashion. In view of the wide scope of the Mitsunobu reaction, we feel that many more important systems with the carbon skeleton of serine and homoserine should be preparable through this basic chemistry in any site‐directed isotopically labeled form. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)