The high specificity in the recognition and specific binding of potassium ion by the depsipeptide valinomycin (VM) is exploited for its recognition and quantitation using both circular dichroism (CD) and optical rotation dispersion (ORD). The specific rotation of VM is comparably small (2.34 deg ml g⁻¹ cm⁻¹), so that an 8 µM (= 8.89 mg ml⁻¹) solution of VM in 95% ethanol rotates polarized light of Λ = 426 nm passing a 2 cm cuvette by 0.076° only. It is shown, however, that VM undergoes large changes in both ORD and CD on binding to potassium ion. VM, potassium ion and the anionic dye merocyanine 540 form a ternary complex (VM/K/MC) which displays an induced CD with a positive maximum at 488 nm and a negative maximum at 470 nm. The ternary complex also displays fluorescence that is weaker by about 30% when compared to that of the dye alone. The induced CD of the ternary complex is interpreted in terms of the large conformational change which VM is known to undergo on binding potassium ion, thereby forming the prerequisite for a van der Waals interaction between its outwardly directed lipophilic domains and the lipophilic domains of the anionic dye. The method is likely to be applicable to the fluorescent detection of all kinds of ions for which chiral receptors are known, e.g. in studies on the role of ions in biological systems including ion channels.