An oxygen separation tube with a composite structure in which a dense membrane is formed on a porous body was successfully fabricated using a newly developed mixed conducting oxide, St-Co0.09Nb0.1O3-delta (SCN). Using the tube obtained, it was confirmed that high-purity oxygen was separated from compressed air. The permeation rate reached 9 cm(3) min(-1) cm(-2) at an oxygen collection rate of 20% under the condition of 900 degrees C and 10 atm air. It was also suggested that the oxygen separation rate followed the theoretical equation assuming a diffusion-limited process even in thin-membrane regions of, for example, 50 mu m. From the measured permeation rate, oxide ionic conductivity was estimated to be around 100 S/m at 900 degrees C. Furthermore, stable oxygen separation from higher air pressure up to 20 atm was successfully observed, which proved the high stability of SCN as well as its superior permeability. The reliability of the fabricated separation tube was also confirmed through a long-term separation test over a half-year period conducted under 900 degrees C, 10 atm air. (C) 2007 Elsevier B.V All rights reserved.