We show that the N-methylimidazole-based selone Im(OH)Se having an NCH2CH2OH substituent has the remarkable ability to degrade methylmercury by two distinct pathways. Under basic conditions, ImOHSe converts MeHgCl into biologically inert HgSe nanoparticles and Me2Hg via the formation of an unstable intermediate (MeHg)(2)Se (pathway I). However, under neutral conditions, in the absence of any base, Im(OH)Se facilitates the cleavage of the Hg-C bond of MeHgCl at room temperature (23 degrees C), leading to the formation of a stable cleaved product, the tetracoordinated mononuclear mercury compound (Im(OH)Se)(2)HgCl2 and Me2Hg (pathway II). The initial rate of HgC bond cleavage of MeHgCl induced by Im(OH)Se is almost 2-fold higher than the initial rate observed by Im(Me)Se. Moreover, we show that Im(Y)Se (Y = OH, Me) has an excellent ability to dealkylate Me2Hg at room temperature. Under acidic conditions, in the presence of excess Im(Y)Se, the volatile and toxic Me2Hg further decomposes to the tetracoordinated mononuclear mercury compound [(Im(Y)Se)(4)Hg](2+). In addition, the treatment of Im(OH)Se with MeHgCys or MeHgSG in phosphate buffer (pH 8.5) afforded water-soluble Hg(SeS) nanoparticles via unusual ligand exchange reactions, whereas its derivative Im(OM)eSe or Im(Me)Se, lacking the NCH2CH2OH substituent, failed to produce Hg(SeS) nanoparticles under identical reaction conditions.