Solid state reactions and reconstructive phase transitions exhibit more or less a large hysteresis between reaction temperatures taken from DTA-heating and -cooling curves. For ternary lanthanide chlorides the equilibrium temperatures could be obtained by e.m.f.-measurements in galvanic chlorine cells for solid electrolytes using A+-ion (A = alkaline metal) conducting diaphragms. By quenching, high-temperature phases can often be transformed to metastable roomtemperature phases. In this case the equilibrium state must be established by annealing at sufficiently high temperature, or it must be tried to synthesize the compound in its stability range from suitable precursor systems.