The solvent acid‐base properties of melts containing 50–70 mole per cent at temperatures of 175°–355°C are fully described by the use of three equilibrium constants At 175°C the K values on the mole fraction scale are , respectively. The ΔH and ΔS values for each equilibrium were found to be temperature independent and calculated values for previously unreported formation constants for the liquid , , and species are reported. Emf data for the aluminum electrode determined as a function of the mole per cent AlCl3, are used to evaluate the equilibrium constants. In the calculations it was necessary to use a variable activity coefficient when the mole per cent exceeded 50%. An experimental nondestructive in situ procedure for the determination of the exact composition and pCl of any melt is described. This procedure is essential when working in volatile melts.