Abstract: The formation and solvent extraction conditions of bis(neocuproine)-copper (Ⅰ)·Ⅰ^- ion-pair as well as the composition of ion-pair and its effect on determination sensitivity were studied. Furthermore; we investigated origin of the absorption peaks and the composition of ion-pair was proved by using the solvent extraction spectrophotometry. It was applied to the determination of trace iodide in normal human serum and boiled kelp solution. The results are satisfactory.