High purity raw materials are used for synthesizing La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF6428) powders to reduce the effect of impurity phases on oxygen permeability of the corresponding membranes. The as-synthesized LSCF6428 powders require a sintering temperature above 1180 degrees C to achieve membrane density over 90%. Ball milling of the powders increases the membrane sintering. It also increases oxygen permeation flux from 0.37 to 0.43 ml cm(-2) min(-1) at 950 degrees C for the membranes sintered at 1100 degrees C. A decrease in oxygen permeation fluxes with the further increase in sintering temperature is observed for the membranes with ball-milled starting powders, accompanied by an obvious increase in grain size. It suggests, at low level of impurity phases, the grain boundaries facilitate the oxygen diffusion. The combination of ball milling of the starting powders and a sintering temperature of 1100 degrees C is optimal to achieve high oxygen permeability of LSCF6428 membranes with improved purity.