The biomimetic oxidation of metribuzin, a pre-and post-emergence herbicide with hydrogen peroxide catalyzed by 5, 10, 15, 20-tetraarylporphyrinatoiron (III) chlorides [TAPFe (III) Cl], has been studied yielding 6-t-butyl-3-methylthio-1, 2, 4-triazine-5 (4H)-one, 4-amino-6-t- butyl-3, 5 (2H, 4H)-dione and 6-t-butyl-1, 2, 4-triazin-3, 5 (2H, 4H)-dione under various reaction conditions.