Abstract Catechin is a frequent constituent of condensed tannins in plants. H 2O 2 generated during the autoxidation and enzymic oxidation of catechin was assayed by the very rapid increase in oxidation products, to a stable end point within seconds, brought about by addition of an excess of horseradish peroxidase (HRP) in the absence of exogenous H 2O 2. Production within minutes of measurable amounts of H 2O 2 by enzymic oxidation of catechin was also confirmed by reaction with guaiacol and HRP. It was shown that mushroom tyrosinase and potato phenolase generated much more H 2O 2 than could be accounted for by coincident autoxidation. HRP alone, without addition of exogenous H 2O 2, was also able to cause more oxidation of catechin than autoxidation. The generation of H 2O 2 by tyrosinase-catalysed oxidation was partly inhibited by superoxide dismutase (SOD) and the subsequent reaction with HRP could be entirely abolished by catalase. Reduction of nitro blue tetrazolium and cytochrome c was brought about by tyrosinase-catalysed oxidation of catechin and was partly inhibited by SOD, indicating generation of superoxide during catechin oxidation. Combinations of tyrosinase and HRP synergistically catalysed oxidation of catechin. In addition, HRP was also able to oxidize chlorogenic acid and gallic acid without exogenous H 2O 2. Not every phenolic species, however, showed the same capacity for generating H 2O 2 during enzymic oxidation.