Abstract: The mechanism of chelate and synergistic extraction of Pb⁺⁺,Ni⁺⁺,Mn⁺⁺,Co⁺⁺ Cd⁺⁺⁺,Ca⁺⁺, Mg⁺⁺ and La⁺⁺⁺ ions with l-phenyl-3-methyl-4-benzoyl-pyrazolone-5(HPZL)-benzene and HPZL-TBP-benzene solution were investigated by the two-phase titration method. The composition of the extracted chelates and synergistic complexes and their stability constants have been determined. The experimental results showed that for the metal ions which are extracted strongly the synergistic effects are weaker, while the metal ions extracted weakly exhibit stronger synergistic effect. It was also found that there exist some relation between the coordination number and the cova-lent character of the coordinate bond. If the coordinate bond has more cova-lent character, the coordination number tends to take the smaller value and the synergistic effect is weaker. On the other hand, if the coordinate bond has more ionic character, the coordination number tends to take the larger value and the synergistic effect is stronger.