General features of peroxidizing compounds are protoporphyrinogen-IX oxidase (Protox) inhibition, light-induced ethane formation, decreased chlorophyll content, growth inhibition of Scenedesmus acutus and root-growth inhibition of Echinochloa utilis. The first two parameters can be conveniently measured in a peroxidizing assay. The two parameters of peroxidizing phytotoxicities were measured with two series of compounds, i.e. 4-(4-bromophenyl)-1, 2-dialkyl (or polymethylene)-1, 2, 4-triazolidines and 5-(4-bromophenylimino)-3, 4-dialkyl (or polymethylene)-1, 3, 4-thiadiazolidines.The order of peroxidizing activity among the 4-(4-bromophenyl)-1, 2-dialkyl (or polymethylene)-5-thioxo-1, 2, 4-thiazolidin-3-ones was as follows: 1, 2-tetramethylene analogue > 1, 2-trimethylene analogue > 1, 2-pentamethylene analogue> 1, 2-dimethyl analogue> 1, 2-dimethyl analogue> 1, 2-dipropyl analogue > 4-(4-bromophenyl)-5-thioxo-1, 2, 4-triazolidin-3-one.The similar activity order was obtained also in the series of other 4-(4-bromophenyl)-1, 2-dialkyl (or polymethylene)-1, 2, 4-triazolidine-3, 5-diones (or dithiones) and 5-(4-bromophenylimino)-3, 4-dialkyl (or polymethylene)-l, 3, 4-thiadiazolidin-2-ones (or thiones). Furthermore, a structure-activity study revealed that the Protox inhibition by 4-(4-bromophenyl)-1, 2-dialkyl (or polymethylene)-5-thioxo-1, 2, 4-triazolidin-3-ones correlated well with the molecular size of compound.