The phase diagram of the system RbCl/ErCl3 was investigated by means of DTA and the system CsCl/ErCl3 was re-investigated. The existence of the caesium compounds Cs3ErCl6 (dimorphic), Cs2ErCl5, Cs3Er2Cl7 and CsEr2Cl7 was confirmed. In the system RbCl/ErCl3, there are two dimorphic, congruently melting compounds, Rb3ErCl6 and RbEr2Cl7. The incongruently melting compound Rb2ErCl5 (Cs2DyCl5 structure) is stable at temperatures higher than 348 degrees C. The thermodynamic stabilities of the compounds were determined by solution calorimetry and e.m.f. measurements in a galvanic chlorine cell for solid electrolytes.