The stability constants of the complexes of rare earth elements – La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu with arylhydrazones of benzoylacetone have been determined by the method of potentiometric titration, which change in the following sequence: Lu > Yb > Tm > Er > Ho > Dy > Tb > Gd > Eu > Sm > Nd > Pr > Ce > La. The stability of the complexes depends on the electron-donor and electron-acceptor character of the functional groups included in the ligands: –Cl>–SO₃H>–NO_2.