2-(1,3-Dimethylimidazolidinio)diselenocarboxylate (16) was obtained in 48% yield as thermally stable, dark green crystals by reaction of 2-methylene-1,3-dimethylimidazolidine (7) with Se2Cl2 in the presence of Et3N. The reaction of 16 with elemental sulfur gave 2-(1,3-dimethylimidazolidinio)dithiocarboxylate (20) in 95% yield by exhaustive selenium-sulfur exchange, whereas the use of a limited amount of sulfur allowed the-preparation of 2-(1,3-dimethylimidazolidinio)selenothiocarboxylate (19) as dark red crystals. X-ray crystallographic analyses revealed that the planes of the carbenium ion and CX2- (X = S, Se) parts of 16 and 20 are nearly perpendicular to each other. The observed geometries (including bond angles and lengths) of 16 and 20 were reproduced satisfactorily by ab initio calculations (B3LYP/6-31G* level). Discussion is also made on the structures of 16, 19, and 20 on the basis of spectroscopic data (NMR, IR, and UV/vis). Methylation of 16 with Mel furnished the carbenium iodide (26a), which possesses a diselenoester moiety, as thermally labile, green crystals. The reaction of 16 with two molar amounts of DMAD provided the 1:2 adduct (30a) in 67% yield. The reaction of 16 with N-[(p-tolylsulfonyl)imino]phenyliodinane gave the inner sale (35) in 68% yield with loss of selenium atom from the initial adduct (33).