A systematic study of the chemical interaction of Ba 2YCu 3O 6+ y and Gd 3NbO 7 was conducted under two processing conditions: purified air (21% p o 2 ), and 100 Pa p o 2 (0.1% p o 2 ). Phases present along the pseudo-binary join Ba 2YCu 3O 6 z and Gd 3NbO 7 were found to be in two five-phase volumes within the BaO − 1 2 Y 2 O 3 − 1 2 Gd 2 O 3 − Nb 2 O 5 − CuO y system. Three common phases that are present in all samples are (Y,Gd) 2Cu 2O 5, Ba(Y,Gd) 2CuO 5 and Cu 2O or CuO (depending on the processing conditions). The assemblies of phases can be categorized in three regions, with Ba 2YCu 3O 6+ y : Gd 3NbO 7 ratios of (I)<5.5:4.5; (II)=5.5:4.5; and (III)>5.5:4.5. The lowest melting temperature of the system was determined to be ≈938 °C in air, and 850 °C at 100 Pa p o 2 . Structure determinations of two selected phases, Ba 2(Gd x Y 1− x )NbO 6 ( Fm3¯ m, No. 225), and (Gd x Y 3− x )NbO 7 ( C222 1, No. 20 and Ccmm, No. 63), were completed using the X-ray Rietveld refinement technique. Reference X-ray powder diffraction patterns for selected phases of Ba 2(Gd x Y 1− x )NbO 6 ( x=0.2, 0.4, 0.6, and 0.8) and (Gd x Y 3− x )NbO 7 ( x=0.6, 1.2, 1.8, 2.4 and 3) have been prepared for inclusion in the Powder Diffraction File (PDF). Crystal structure for (Gd x Y 3− x )NbO 7 showing the partial layered feature. The alternate stacking of distorted NbO 6 octahedra and (Gd,Y)O 7 polyhedra are illustrated. The (Gd,Y)O 8 polyhedra are omitted for clarity.