Peroxidizing herbicides, as inhibitors of protoporphyrinogen IX oxidase (protox), cause accumulation of protoporphyrin IX and lead to a rapid phytotoxic degradation of plant cell membranes in the light. Certain 5-arylimino-3,4-tetramethylene-1,3,4-thiadiazolidin-2-ones are peroxidizing compounds. They are isomerized to the corresponding more active triazolidines with glutathione S-transferase (GST) in the presence of reduced glutathione (GSH). The rate of isomerization depends on the arylimino moiety and the presence of carbonyl or thiocarbonyl groups at the thiadiazolidine ring. The following compounds and their isomeric triazolidines were synthesized: 5-(4-Bromophenyl)-3,4-dialkyl-1,3,4-thiadiazolidin-2-ones (I), 5-(4-brom ophenyl)-3,4-dialkyl-1,3,4-thiadiazolidine-2-thiones (II), 5-(4-brom ophenyl)-3,4-alkylene-1,3,4-thiadiazolidin-2-ones (III), 5-(4-brom ophenyl)-3,4-alkylene-1,3,4-thiadiazolidine-2 -thiones (IV). The phytotoxic activities of these compounds were investigated using Scenedesmus acutus and protoporphyrinogen IX oxidase isolated from corn (Zea mays) etioplasts. These compounds are peroxidizing compounds, and the triazolidines showed higher activity than the thiadiazolidines. Compounds I and III were isomerized to the corresponding triazolidines, while compounds II and IV were not or scarcely converted into their isomers with GST in the presence of GSH . Seemingly, the bulkiness of alkylene or dialkyl groups at the heterocycle influences the isomerization of carbonyl-substituted thiadiazolidines.