The cubic perovskite La0.2Sr0.8Cu0.4Co0.6O3-x has high electronic and ionic conductivity at high temperatures. The electronic conductivity initially increases with increasing temperature but then decreases due to loss of oxygen atoms from the structure. The oxygen permeability measured on disc membranes is comparable to values reported for other cobalt-containing perovskite oxides with high strontium content. The permeation flux decreases with time due to a transformation of the disordered cubic perovskite to an ordered structure related to Brownmillerite. This ordered structure is stable below 975 degrees C in an oxygen partial pressure pO(2) = 0.01 atm. Membranes that have been maintained at high temperature in an oxygen partial pressure gradient for >400 h show decomposition to a multi-phase assemblage that includes non-perovskite strontium cuprates. This decomposition may also contribute to the flux decline.