Certain 5-(arylimino)-3,4-tetramethylene-1,3,4-thiadiazolidin-2-ones (thiadiazolidines) are peroxidizing bleaching herbicides which interrupt chlorophyll biosynthesis, inhibit the activity of protoporphyrinogen oxidase, lead to accumulation of protoporphyrin IX, and induce ethane formation in the light. The same effects are caused by their isomers, the 4-aryl-1,2-tetramethylene-l,2,4-triazolidin-3-one-5-thiones (triazolidines). Couples of thiadiazolidines and corresponding triazolidine isomers were synthesized. Thiadiazolidines with a 4-bromophenylimino, 4-chlorophenylimino, 4-chloro-2-methylphenylimino, 4-chloro- 2-fluorophenylimino, 4-chloro-2-fluoro-5-propargyloxyphenylimino and 4-chloro-2-fluoro- 5-isopropoxyphenylimino moiety were converted to the corresponding triazolidines both with Echinochloa seedlings or a spinach homogenate present, depending on the 5-arylimino moiety. The 5-[4-(chlorobenzyloxy)phenylimino]-3,4-tetramethylene-1,3,4-thiadiazolidin-2- one analogue did not convert to the corresponding triazolidine under both conditions. Thiadiazolidines as well as triazolidines having a 4-chloro-2-fluoro-5-methoxycarbonylmethylthiophenyl moiety were converted to an unidentified compound whose structure is assumed to be 4-(4-chloro-2-fluoro-5-carboxymethylthiophenyl)-1,2-tetramethylene-1,2,4- triazolidin-3-one-5-thione. Apparently, the general conversion mechanism is caused by enzymatic hydrolysis of thiadiazolidines to an unstable intermediate which rapidly and spontaneously changes to the corresponding triazolidine isomer.