Absolute rate constants have been measured for the reaction of the anionic conjugate base of metiazinic acid, MZ–, in the pH range 8–11, with hydrated electrons (k 1.5 × 109 l mol–1 s–1), and for the oxidation of MZ– by OH˙, Br2–˙, (SCN)2–˙, CO3–˙, TI(OH)2, N3, ˙˙CH2CHO, (CH3SCH3)2+˙, CH3SSCH3+˙ and lipoate+˙. The k values for these oxidations are found to be in the 109–1010 l mol–1 s–1 range, except for k(˙CH2CHO + MZ–) 2 × 108 l mol–1 s–1. The oxidation of metiazinic acid leads to MZ±˙ radical zwitterions with yields ranging from 50 to 100% with the minimum value referring to the oxidation by ˙CH2CHO. The MZ±˙ radical zwitterion is characterized by two strong optical absorption bands with maxima at 530 (ε 11 000 l mol–1 cm–1) and 270 nm (51 500). Other primary oxidation products are indicated, but only a transient absorption around 350 nm (OH˙ adduct and/or hydrogen abstraction product) could qualitatively be identified in the reaction of metiazinic acid with hydroxyl radicals.