4-Hydroxyphenylpyruvic acid was oxygenated with various complexes of oxygen with Fe2+, superoxide ion, hydroperoxide anion, triplet and singlet oxygen. Oxidation occurred giving varying amounts of the 4-hydroxy derivatives of benzaldehyde, benzoic acid, phenol, phenylacetic acid and 4-carboxymethylquinone, but no homogentisic acid. 4-Hydroxyphenylperacetic acid was prepared and shown not to undergo self-oxidation. Its ferrous salt decomposed to 4-hydroxybenzyl alcohol. It is concluded that the alpha-keto carboxylic function is the site of oxygenation in the enzyme process and that a trioxalanone rather than a peracid intermediate may be implicated.