Cobalt(II) chloride is an inorganic compound, a salt of cobalt and chlorine, with the formula CoCl
2. The compound forms several hydrates CoCl2·nH2O, for n = 1, 2, 6, and 9. Claims of the formation of tri- and tetrahydrates have not been confirmed.The anhydrous form is a blue crystalline solid; the dihydrate is purple and the hexahydrate is pink. Commercial samples are usually the hexahydrate, which is one of the most commonly used cobalt salts in the lab.

At room temperature, anhydrous cobalt chloride has the cadmium chloride structure (CdCl
2) (R3m) in which the cobalt(II) ions are octahedrally coordinated. At about 706 °C (20 degrees below the melting point), the coordination is believed to change to tetrahedral.[2] The vapor pressure has been reported as 7.6 mmHg at the melting point.[6]

Cobalt chloride is fairly soluble in water. Under atmospheric pressure, the mass concentration of a saturated solution of CoCl
2 in water is about 54% at the boiling point, 120.2 °C; 48% at 51.25 °C; 35% at 25 °C; 33% at 0 °C; and 29% at −27.8 °C.[4]

Diluted aqueous solutions of CoCl
2 contain the species [Co(H2O)6]2+, besides chloride ions. Concentrated solutions are red at room temperature but become blue at higher temperatures.[7]

The crystal unit of the solid hexahydrate CoCl
2•6H2O contains the neutral molecule trans-CoCl2(H2O)4 and two molecules of water of crystallization.This species dissolves readily in water and alcohol.

The anhydrous salt is hygroscopic and the hexahydrate is deliquescent.[citation needed] The dihydrate, CoCl2(H2O)2, is a coordination polymer. Each Co center is coordinated to four doubly bridging chloride ligands. The octahedron is completed by a pair of mutually trans aquo ligands.