A trivalent metal ion-selective electrode (ISE) consisting of bis(diphenylphosphoryl)methane as an ionophore was developed for the determination of formation constants with organic ligands. The ISE prepared exhibited Nernstian response to the concentration of Eu3+ in the test solutions. Even in the presence of carboxylic acids such as malonic acid in samples, the obtained potential values were found to be stable and reproducible during the measurement time. The present formation constants of Eu with carboxylates were in good agreement with the reported values.