Subsolidus equilibria in air in the RuO2-CeO2 and RuO2-Y2O3-CeO2 systems were studied with the aim of obtaining information on possible interactions between the Y2Ru2O7-based cathode and the CeO2-based solid electrolyte in solid oxide fuel cells (SOFC). In the RuO2-CeO2 system, no binary compound and no solid solubility were detected. In the RuO2-Y2O3-CeO2 system, no ternary compound was found. The tie line is between Y2Ru2O7 and CeO2.