Selenium monochloride is an inorganic compound with the formula Se2Cl2. Although it is called selenium monochloride, a more descriptive name might be diselenium dichloride. It is a reddish-brown, oily liquid that is hydrolyses slowly. It exists in chemical equilibrium with SeCl2, SeCl4, chlorine, and elemental selenium. Selenium monochloride is mainly used as a reagent for the synthesis of Se-containing compounds. Selenium monochloride exhibits C2 molecular symmetry, similar to hydrogen peroxide. The bond length of the Se-Se bond is predicted to be 2.253 Å and the Se-Cl bonds are both 2.227 Å.