Guaiacol is a phenolic natural product first isolated from Guaiac resin and the oxidation of lignin. Guaiacol is readily oxidized by the heme iron of peroxidases including the peroxidase of COX enzymes. It can therefore serve as a reducing co-substrate for COX reactions. The one-electron oxidation product of guaiacol is a dimer absorbing at 470 nm with an extinction coefficient of 26,600. Two moles of guaiacol are oxidized for each mole of hydroperoxide reduced by the peroxidase. The resulting guaiacol chromophore can be used for the colorimetric determination of hydroperoxidase activity.