It exists in two polymorphs (crystalline forms).

The compound is used to prepare other osmium complexes.

It was first reported in 1909 as the product of chlorination of osmium metal.

[1] This route affords the high temperature polymorph:[2] This reddish-black polymorph is orthorhombic and adopts a structure in which osmium centres are octahedrally coordinated, sharing opposite edges of the OsCl6 octahedra to form a chain.

[3] A brown, apparently cubic polymorph forms upon reduction of osmium tetroxide with thionyl chloride:[4] Osmium tetraoxide dissolves in hydrochloric acid to give the hexachloroosmate anion: