Diiodomethane or methylene iodide is liquid halomethane insoluble in water, soluble in ether and alcohol. It has a relatively high refractive index of 1.741. Fresh diiodomethane is a colorless liquid. However, exposure to light causes it to slowly decompose and liberate iodine, which colours it brownish.
With its high specific gravity, it is used for the separation of minerals or for determination of specific gravity of mineral particles. Due to its possible toxicity, it should be replaced with safer heavy liquids. It is also used as a solvent and a chemical intermediate for example in the Simmons-Smith reaction.