Abstract: Hydrolyiic polymerization of Eu³⁺ and Gd³⁺ion was studied by potenliomclry at 25 ±0. 1℃.The concentration of rare earth ions ranged from 0. 1Mto 0.6Mand the ionic strength was kept at 2M NO₃^- with NaNO₃ by self medium method. The composition and the hydrolysis constants of hydrolytic products were calculated by two different methods, the results of which agreed with each other fairly well.The hydrolytic state of Eu³⁺ and Gd⁵⁺ion was shown to be the same as that of Sm³⁺, but the degree of polymerization of the former two ions was higher.They formed poly-nuclear species. M₂(OH)₂⁴⁺,M₃(OH)₄⁵⁺ and M₄(OH)₆⁶⁺ besides mononuclear species.