Abstract: So far as the authors are aware, the stability constants of the rare earth ions with some inorganic ligands reported in the literature are quite inconsistent with one another.In this paper, the complex formation of all the rare earth ions (besides Ce and Pm) has been studied by ion exchange method at 25℃ in NaClO₄ (2M) ionic medium. The stability constants were calculated by a special computer program written for this purpose in Beida 6912 language. In the case of thiocyanate the complexes were RECNS²⁺ , RE(CNS)₂⁺, RE(CNS)₃, but in the case of nitrate, almost only RENO₃²⁺. We use Y₍₁₎ to express the ratio (C⁰_RE)/(RE³⁺) when (L^-) = 1M, where (C⁰_RE), (RE³⁺), (L^-) represent the concentrations of the total metals, the free metals, and the total ligands respectively. The values of Y₍₁₎ were irregular but had a tendency of increasing with the increase of the atomic number in thiocyanate system and decreasing with the increase of the atomic number in nitrate system. The value of Y₍₁₎ of yttrium was the smallest both in thiocyanate and nitrate systems.