We show that the N-methylimidazole-based selone ImOHSe having an N−CH2CH2OH 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)2Se (pathway I). However, under neutral conditions, in the absence of any base, ImOHSe facilitates the cleavage of the Hg−C bond of MeHgCl at room temperature (23 °C), leading to the formation of a stable cleaved product, the tetracoordinated mononuclear mercury compound (ImOHSe)2HgCl2 and Me2Hg (pathway II). The initial rate of Hg−C bond cleavage of MeHgCl induced by ImOHSe is almost 2-fold higher than the initial rate observed by ImMeSe. Moreover, we show that ImYSe (Y = OH, Me) has an excellent ability to dealkylate Me2Hg at room temperature. Under acidic conditions, in the presence of excess ImYSe, the volatile and toxic Me2Hg further decomposes to the tetracoordinated mononuclear mercury compound [(ImYSe)4Hg]2+. In addition, the treatment of ImOHSe with MeHgCys or MeHgSG in phosphate buffer (pH 8.5) afforded water-soluble Hg(SeS) nanoparticles via unusual ligand exchange reactions, whereas its derivative ImOMeSe or ImMeSe, lacking the N−CH2CH2OH substituent, failed to produce Hg(SeS) nanoparticles under identical reaction conditions.