AbstractCatechol oxygen atoms can be exchanged with oxygen‐18 atoms in [18O]‐H2O under acidic conditions. The catechol oxygen atoms in L‐3, 4‐dihydroxyphenylalanine both underwent significant exchange with water when heated in 3 N HC1 at 150 °C for 20 days. Under the same conditions of heat and strong acid, vanillylmandelic acid was degarded to homovanillic and dihydroxyphenylacetic acid in two hours. Exchange of the catechol oxygen with the solvent occurred simultaneously with degradation. This degradation of vanillylmandelic acid formally involves reductive cleavage of the aliphatic hydroxyl in a reaction which is probably general for compounds of similar structure. An analogous, although slower, degradation of dihydroxyphenyllactic acid to dihydroxyphenylpropionic acid accompanied by catechol oxygen atom exchange was observed. Direct exchange of catechol oxygen atoms and degradation of appropriate precursor molecules are two facile methods for the preparation of oxygen‐18 labelled compounds containing the catec