The complexation of gallium(III) with 2,2′,3,4-tetrahydroxy-3′- sulfo-5′-nitrobenzene in the presence of and without 1,10-phenanthroline was studied. In the presence of 1,10-phenanthroline, a mixed-ligand complex with the component ratio 1:2:1 and the stability constant logβ = 15.5 ± 0.2 is formed. The effect of pH, time, temperature, and the concentration of components on the formation of the binary and mixed-ligand complexes of gallium was studied. A procedure was developed for the photometric determination of gallium(III) in the presence of aluminum (III). © 2007 Pleiades Publishing, Ltd;