Chemical interaction between the Ba 2YCu 3O 6+ x superconductor and the CeO 2 buffer layers employed in coated conductor architectures has been modeled experimentally by investigating phase equilibria on the Ba 2YCu 3O 6+ x CeO 2 join at p O 2 = 100 Pa . This join is actually a non-binary join within the BaO Y 2O 3 CeO 2 CuO x quaternary system. At an approximate mole ratio of Ba 2YCu 3O 6+ x : CeO 2 = 40 : 60, a phase boundary was found to separate two four-phase regions. At the Ba 2YCu 3O 6+ x -rich side of the join, the four-phase region consists of Ba 2YCu 3O 6+ x , Ba(Ce 1− z Y z )O 3− x , BaY 2CuO 5, and Cu 2O; at the CeO 2 rich side, the four phases were determined to be Ba(Ce 1− z Y z )O 3− x , BaY 2CuO 5, Cu 2O and CeO 2. At 810 °C and p O 2 = 100 Pa , there appears to be negligible solid solution formation of the types Y 1− z Ce z O 3− x and Ce 1− z Y z O 2− x . The minimum melting temperature along the Ba 2YCu 3O 6+ x CeO 2 join was determined to be ≈860 °C. As part of this study, phase diagrams of the subsystems CeO 2 Y 2O 3 CuO x , BaO CeO 2 CuO x , and BaO Y 2O 3 CeO 2 were also determined at 810 °C under 100 Pa p O 2 . The Y 2O 3 CeO 2 CuO x diagram does not contain ternary phases and shows a tie-line from Y 2O 3 to the binary phase Y 2Cu 2O 5− x . Similarly, the BaO CeO 2 CuO x diagram contains no ternary phases, but has four tie-lines originating from BaCeO 3 to Ba 2CuO 3+ x , BaCuO 2+ x , BaCu 2O 2+ x and CuO x . The BaO Y 2O 3 CeO 2 system contains one ternary phase, the solid solution Ba(Ce 1− z Y z )O 3− x ( 0 ⩽ z ⩽ 0.13 ), which crystallizes with the orthorhombic space group Pmcn (No. 62). Neutron Rietveld refinement of Ba(Ce 0.94Y 0.06)O 2.84 [ Ba ( Ce 3 + 0.26 Ce 4 + 0.68 Y 0.06 ) O 2.84 ] gives lattice parameters of a = 8.7817 ( 4 ) Å , b = 6.2360 ( 4 ) Å , c = 6.2190 ( 3 ) Å , V = 340.57 Å 3 , Z = 4 . The structure consists of distorted corner-shared (Ce,Y)O 6 octahedra that are tilted with respect to each other. Ba was found to have 9 nearest neighbors (distances < 3.5 Å). Reference X-ray patterns of Ba(Ce 0.90Y 0.10)O 2.95 and Ba(Ce 0.87Y 0.13)O 2.93 were submitted to be included in the Powder Diffraction File (PDF).