The thermally induced segregation of Mg in a 1% Mg/Al alloy in the temperature range 225 to 350 degrees C has been studied by means of Auger electron depth profiling. An activation energy for the process has been determined and a linear relationship was found between the amount of Mg enrichment in the surface region and temperature. The results suggest that the segregation is chemically driven by the greater affinity of Mg for O.