The herbicide hexazinone (3-cyclohexyl-6-(dimethylamino)-1-methyl,3,5-triazine-2,4 (1H,3H)-dione) is a 1,3,5-triazine derivative which exists in a single cyclic tautomeric form. In the presence of water its azomethine bond is covalently hydrated (K-d = 0.79, in acetonitrile as a predominant co-solvent) and protonated (pK(a) = 1.2). Following dehydration, the protonated form of the azomethine bond undergoes a two-electron reduction in wave I-1. The decrease in current at pH > 2.5 results from the decrease in the rate of protonation (k(r) = 6.3 x 10(6) mol(-1) s(-1)). The current at pH 2 to 4 is smaller than theoretical due to a competitive hydration. The two-electron reduction product formed in waves undergoes elimination of dimethylamine. The resulting azomethine bond in the product of elimination is reduced in wave I-2. The oxidized form of hexazinone undergoes an acid-catalyzed ring opening at pH < 1.3. The acid catalysis is attributed to a protonation of the hydrated form of the heterocycle. The formyl group of the open-chain form is reduced in a more negative wave I-1a. Ac polarography indicated adsorption of the oxidized form of hexazinone, dc polarography and voltammetry show also an adsorption of its reduced form.