It was discovered that 2,3-bis-(2-methoxy-phenyl)-5-phenylamino-[1,2,4]-thiadiazolium bromide (1), a 1,2,4-thiadiazolium derivative, could be reduced to the corresponding imidoylthiourea, 1-[(2-methoxy-phenyl)-(2-methoxy-phenylimino)-methyl]-3-phenyl-thiourea (3), by some biologically interesting reducing reagents including glutathione, cysteine, and ascorbic acid. The reduction also occurred in Sprague-Dawley rat and Yorkshire swine plasma, suggesting that thiol containing biological molecules existing in the plasma are mainly responsible for this reaction. A facile method for preparation of 3 from 1 was established by using 2-thioethanol as reaction reagent as well as solvent. The structure of 3 was fully characterized using nuclear magnetic resonance (NMR) and mass spectrometry with electrospray ionization source (ESI-MS). Those new findings could shed light on the development of 1,2,4-thiadiazolium derivatives for their potential pharmaceutical applications.