Membrane reactors enable synthesis-gas production from methane and air while avoiding the need for separation of the nitrogen either before or after the reaction. A stable membrane was developed by spray deposition of a dense thin film of La0.5Sr0.5Fe0.8Ga0.2O3 − δ on a high-purity porous α-alumina tube. The oxygen permeation rate at 850°C was 2.5 × 10−7 mol·cm−2·s−1. A quartz tube was placed coaxially around the membrane and the shell filled with a rhodium catalyst. Air was fed to the tube and methane to the shell. At 850°C the methane conversion was 97% and the selectivity to carbon monoxide approached 100%. Rapid radial mixing of the oxygen in the shell is essential to prevent coking and undesirable reactions. The membrane decomposes at 780°C in pure CH4, but remains stable up to 970°C in a mixture of 90-mol % CH4 and 10-mol % CO2.