A dynamic combinatorial library of metal–dye complexes was obtained by reacting aqueous solutions of the dyes Methyl Calcein Blue, Arsenazo I, and Xylenol Orange with CuCl2 and NiCl2. The mixture gave a characteristic UV-Vis response upon addition of the peptide hormones angiotensin I and angiotensin II. This allowed distinguishing pure samples of peptide hormones from mixtures. The discriminatory power of the sensor was enhanced when the several UV/Vis measurements were performed during the equilibration process of the library.