Abstract: A study on the equilibrium between ferric and ferrous species in chloride melts has been made for better understanding of the behavior of iron species in magnesium electrolysis and moltensalt chlorination. At PCl₂=101.325kPa. and experimental temperatures, the apparent equilibrium constant of the reaction can be given. At PCl₂=0 and experimental temperatures, FeCl₃ decomposed completely. It has been found that the measured values of K were in good agreement with the expected ones from the regression equations. The composition of melts, partial presure of chlorine and temperature had important effect on K. At the same other conditions, there were the maximunm values of K in system 3, indicating that the current efficiency of magnesium electrolysis would be lower if carnallitc was used as electrolyte, and that the catalytic effect of iron in molten-salt chlorination would be better when the content of KCI in the melts increaesed.