The subsolidus phase relations of the SrO–REO x –CuO (RE=Ce, Pr, Tb) systems were investigated in air. The samples were equilibrated at 1173K. In the SrO–CeO 2 –CuO system, there are neither ternary oxide compounds nor ternary solid solutions. However, the slight solubility of Sr into CeO 2 results in the formation of a two-phase region, where Ce 1−x Sr x O 2−δ is in equilibrium with the Sr 14 Cu 24 O 41−δ phase. The SrO–PrO x –CuO system contains four stable ternary oxide phases: Pr 1.4 Sr 0.4 CuO 4−δ , Pr 7 Sr 8 Cu 2 O x , Pr 1.85 Sr 1.15 Cu 2 O 6−δ and Pr 2−x Sr 1+x Cu 2 O 6−δ (0.8≤x≤0.9). Two binary oxide compounds form ternary solid solutions extending into the pseudo-ternary sections: Pr 2−x Sr x CuO 4−δ (0.00≤x≤0.05) and Sr 14−x Pr x Cu 24 O 41−δ (0.0≤x≤6.0). The Sr 14−x Tb x Cu 24 O 41−δ solid solution (0.0≤x<6.0) is the only ternary phase formed in the SrO–TbO x –CuO system. This system is dominated by the SrTbO 3 phase, which is in equilibrium with all the other compounds.