The title compounds were prepared by conventional solid state reaction and by thermal decomposition of freeze-dried oxalate precursors using different temperature regimes. The thermal decomposition of an appropriately composed oxalate precursor enables the direct synthesis of pure monoclinic CuNb2O6, thermodynamically stable only at theta less than or equal to 700 degrees C, The solid state reaction leads to the orthorhombic columbite type phase or to a mixture of the orthorhombic and monoclinic phases. A new metastable CuTa2O6-trirutile phase was synthesized by thermal decomposition of an oxalate precursor. Above 800 degrees C, the decomposition results in the already known perovskite-like structure. Tetragonal perovskites Ba3Cu1+xNb2-xO9-1.5x and Ba3Cu1+xTa2-xO9-1.5x were synthesized by solid state reaction and by precursor decomposition. The phase formation and homogeneity of the perovskites strongly depends on the reaction conditions.