A ternary complex is formed when 7-(1'-naphthylamine-azo)-8-hydroxyquinoline-5-sulfonic acid (NAOX) occupied the two remaining coordination sites on a lanthanide ion complexed by a hexadentate aminopolycarboxylate ligand. The effects on the fluorescence intensity are studied. The NAOX/EDTA reagent system provides sensitive and selective determination of Lu 3 + and Y 3 + , with the detection limits of 5.0x10 - 7 and 7.0x10 - 7 mol/l, respectively. In addition, the ternary complex can be used for determination of EDTA or other hexadentate aminopolycarboxylic acids.