The oxidation of catechol in the presence of polyphenol oxidase has been investigated using paper chromatographic methods together with the manometric. Under conditions of very low substrate concentration and optimum oxygen uptake the only intermediate which can be detected is a purple-red pigment. At higher substrate concentrations the three isomeric tetrahydroxydiphenyls are formed, together with another compound whose structure is being investigated. These compounds can also be prepared from catechol with inorganic oxidants. The tetrahydroxydiphenyls only consume half the oxygen per aromatic ring compared to catechol and their production in varying amounts at different concentrations of catechol may explain the divergence in reported oxygen uptakes.