Dual-phase composite oxygen transport membranes consisting of 50 vol% Al0.02Zn0.98O1.01 and 50 vol% (ZrO2)(0.89)(Y2O3)(0.01)(Sc2O3)(0.10) were successfully developed and tested. The applicability of the membrane in oxy-fuel power plants schemes involving direct exposure to flue gas was evaluated by exposing the membrane to gas streams containing CO2, SO2, H2O and investigating possible reactions between the membrane material and these gases. The analyses of the exposed composites by x-ray diffraction (XRD), x-ray fluorescence (XRF), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy revealed excellent stability. Additionally, an electrical conductivity measurement over 900 h confirmed that the composite is stable under prolonged exposure to CO2. However, an instability of the dual-phase membrane under oxygen partial pressures below P-O2 similar to 10(-4) atm. was found. Oxygen permeation tests on a 1 mm thick self-standing membrane resulted in an oxygen flux of 0.33 mLN min(-1) cm(-2) at 925 degrees C in air/N-2. Stability tests in CO2 with 3 vol% O-2 demonstrated the potential for the use of 10Sc1YSZ-AZO dual-phase membranes in oxy-combustion processes involving direct exposure to flue gas.