A systematicstudyofthechemicalinteractionofBa2YCu3O6+y and Gd3NbO7 was conductedundertwo processingconditions:purifiedair(21% po2 ), and100Pa po2 (0.1% po2 ). Phasespresentalongthepseudo- binaryjoinBa2YCu3O6z and Gd3NbO7 werefoundtobeintwofive-phasevolumeswithinthe BaO12 Y2O312 Gd2O3Nb2O5CuOy system. Threecommonphasesthatarepresentinallsamplesare (Y,Gd)2Cu2O5, Ba(Y,Gd)2CuO5 and Cu2O orCuO(dependingontheprocessingconditions).The assembliesofphasescanbecategorizedinthreeregions,withBa2YCu3O6+y: Gd3NbO7 ratiosof (I)o5.5:4.5;(II)=5.5:4.5;and(III)45.5:4.5.Thelowestmeltingtemperatureofthesystemwas determinedtobe E938 1C inair,and850 1C at100Pa po2 . Structuredeterminationsoftwoselected phases,Ba2(GdxY1x)NbO6 (Fm3 m, No.225),and(GdxY3x)NbO7 (C2221, No.20and Ccmm, No.63), were completedusingtheX-rayRietveldrefinementtechnique.ReferenceX-raypowderdiffraction patternsforselectedphasesofBa2(GdxY1x)NbO6 (x=0.2,0.4,0.6,and0.8)and(GdxY3x)NbO7 (x=0.6, 1.2, 1.8,2.4and3)havebeenpreparedforinclusioninthePowderDiffractionFile(PDF).