Surface exchange and bulk diffusion resist ionic oxygen transport through a dense mixed ionic-electronic conducting (MIEC) membrane. In the regime of mixed resistances, the outwards oxygen ionic flow rate per unit length from the tube to the shell (F-out) is predicted to exceed that from the shell to the tube (F-in) for equivalent rich and lean oxygen partial pressures. Numerical simulations for typical parameter values predict enhancement factors (F-out / F-in) of about 1.05. This directional dependence is not predicted to occur when either the surface of exchange or bulk diffusion are the controlling resistance. Experimental data are in qualitative agreement with the theoretical predictions. The observed ratio between the outwards and inwards oxygen ionic flow at 1000 degrees C through a (LaSrFeGaO3-delta)-Sr-0.5-Fe-0.5-Ga-0.8-O-0.2 (LSFG) membrane tube was in the range of 1.14 to 1.38. (c) 2006 Elsevier B.V. All rights reserved.