A sharp increase in the electrical conductivity of dual phase Ag/BaBi8O13 composites is observed in compositions with sliver contents between 13 and 17 vol.%. Above this percolation threshold, composite membranes have high oxygen permeability in the temperature range 570 less than or equal to T less than or equal to 700 degreesC. The oxygen flux is 20 times lower for a membrane composition with a conductivity below the percolation threshold. The drop in conductivity in BaBi8O13 at the beta (1) to beta (2) phase transition at 570 degreesC is apparent in the permeation flux data. The overall permeation rate is limited by the surface reaction kinetics.