TMPD is an easily oxidizable compound that serves as a reducing co-substrate for heme peroxidases. TMPD undergoes one-electron oxidation by the heme peroxidase higher oxidation states (compounds I and II) producing a highly colored product that absorbs at 611 nm. Thus, the stoichiometry of oxidation is 2 moles of TMPD oxidized per mole of hydroperoxide reduced by the peroxidase. The extinction coefficient of the oxidized TMPD at 611 nm is 12,200. TMPD is also used for the detection of peroxidases on polyacrylamide gels.