The synthesis of the important diphenolic isoflavone type phytoestrogens starting from the corresponding unprotected phenols and arylacetic acids is discussed. The aryl rings may carry additional alkyl, methoxy, and/or halogeno groups. Intermediate polyhydroxy deoxybenzoins can also be isolated in good yield. Isotopically labeled isoflavone phytoestrogens were prepared for use as internal standards in ion exchange chromatography and GC-MS selected ion monitoring (SIM technique). Traditional methods rely on total synthesis using deuterated starting materials for the preparation of labeled isoflavonoid structures. We have used successfully an application where the H/D exchange is performed within the finished molecular framework, based on the exchange of aromatic protons that are ortho or para to a phenolic OH group. By this method the deuterated products are available in an isotopic purity of 90% or higher.