1-Mesityl-1,3-dihydro-imidazole-2-selone, (seim(Mes))H, may be obtained from 1-mesitylimidazole via (i) deprotonation with (BuLi)-Li-n, (ii) treatment with elemental selenium, and (iii) addition of HCl(aq). Structural characterization of (seim(Mes))H by X-ray diffraction demonstrates that the compound exists as the selone rather than selenol tautomer, a result that is in accord with DFT calculations. Solutions of (seim(Mes))H are oxidized by air to give bis(1-mesitylimidazol-2-yl) diselenide, (seim(Mes))(2). A corresponding investigation of (seim(Me)) H demonstrates that, in contrast to a previous report, the selenium analogue of methimazole exists in the selone form with a structure analogous to that of methimazole. H-1 and Se-77 NMR studies demonstrate that the (seim(R)) groups of the selone (seim(R))H and diselenide (seim(R))(2) undergo facile exchange on the NMR time scale.