Gibbsian segregation has been observed in trinary alloy Al-Li-Mg. The experiment was carried out by means of secondary ion mass spectroscope (SIMS). The experimental results indicated that the lithium concentration on specimen surface reached approximately 100% in the specimen temperature region of 150 to 300 C. The depth profile of Li showed that there was some broadening resulting from recoil implantation by high energy Ar+ ion bombardment. When the specimen temperature exceeded the temperature range in which the lithium enrichment reached the maximum value, beryllium, impurity element in the alloy, segregated to the surface and competed with lithium. Irradiation-induced segregation for Be was also observed in the experiment.