A CO2-stable dual phase membrane of the composition 40 wt % NiFe2O4-60 wt % Ce0.9Gd0.1O2-delta (40NFO-60CGO) was synthesized in three different ways: mixing of the.. starting powders (I) in a mortar and (2) in a ball-mill as well as by (3) direct in situ one-pot sol-gel powder synthesis. Backscattered scanning electron microscopy revealed that the direct one-pot synthesis of 40NFO-60CGO gives the smallest grains in a homogeneous distribution, compared with powder homogenization in the mortar or the ball-mill. The smaller is the grains, the higher is the oxygen permeability. The permeation of the membrane can be improved by coating a porous La0.6Sr0.4CoO3-delta (LSC) layer on the surface of the air side. The dual phase membrane of 40NFO-60CGO prepared by in situ synthesis shows a steady oxygen flux of 0.30 ml/(min cm(2)) over more than 100 h when pure CO2 was used as sweep gas, which indicated that the dual phases membrane is CO2-resistant at least over this 5 days testing period. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 2738-2745, 2011