The intercalation of magnesium chloride into graphite has been studied as a function of the temperature, in the presence of a chlorine gas. The second stage compound has been obtained at 580 degrees C, the average formula of which is CgMgCl(2). By cointercalation of MgCl2 and CoCl2, at 600 degrees C, MSCl(2)/CoCl2 ratio equal to 4.5 and during ten days, the first stage compound C-5.65(MgCl2)(0.97)(CoCl2)(0.03) has been obtained. It only contains traces of CoCl2. A theoretical mechanism of this cointercalation is proposed.