The synergistic extraction of gallium (III) with mixed extractants of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA) as the main extractant and four kinds of oxine derivatives as synergists diluted in toluene or n-heptane was examined. The synergists used were 8-hydroxyquinoline, 8-hydroxyquinaldine, 5-fluoro-8-hydroxyquinoline and 5-chloro-8-hydroxyquinoline. The extraction system using EHPNA and 8-hydroxyquinaldine did not show any synergistic effect. The order of magnitude of synergism was as follows; 5-chloro-8-hydroxyquinoline>5-fluoro-8-hydroxyquinoline>8-hydroxyquinoline. The extraction reaction with mixed extractants can be reasonably interpreted based on the formation of three gallium complexes. The chemical effect of the substituent group of the oxine derivatives as synergists was small. The synergistic extraction mechanism of gallium (III) by the mixed extractants of EHPNA and the oxine derivatives can be related to concentration of the synergist in the organic solution, which is determined by its distribution ratio and acid dissociation constant.