The paper considers the thermodynamics of the metal/ molten-chloride system, with particular reference to the corrosion of metals. It is shown that the prime factors which determine how much corrosion can take place in a given system are the nobility of metal and the stability of the chloride. Also important are the initial redox potential of the melt, the solubility in the melt of the metal produced by the reduction of the chloride, the pressure over the system, the solubility product of the metal oxide, and the presence of impurities, including water—which is considerde in some detail. Kinetics are not considered since the interest is mainly on determining the conditions under which the reaction can be expected to come to a virtual halt and it is assumed that at the temperatures under consideration, the rates of reactions (other than those involving extremely small concentrations) will be much in excess of what can be tolerated for critical applications.