AbstractCerium(III) chelate of 7‐iodo‐8‐hydroxyquinoline‐5‐sulphonic acid(ferron) has been studied spectrophotometrically in aqueous solution at 25°C and at an ionic strength of 0.1M. The formation of this chelate waspH dependent, and the optimumpH range was between 5.5 to 6.0. Its mole ratio of ligand to cerium(III) ion was found to be 2 to 1 stoichiometry and the formation constant, logK, was determined as 10.15. Enthalpy and entropy changes characterizing the formation of the chelate have been calculated as follows: ΔG° =‐13.85kcal mole‐1, ΔH°= −5.03kcal mole‐1, ΔS°=29.56eu mole‐1. By using the wavelength of 370nm, determination of trace amount of cerium(III) ion with the sensitivity o