Abstract: The extraction of terbium (Ⅲ) from nitric acid solutions with mixtures of 1-phenyl-3-methyl-4-benzoyl-pyrazolone-5(PMBP)and dibutyl butylphos-phonate (DBBP), tri-n-butyl phosphate (TBP) in chlorobenzol has been investigated.The formulas of the extracted species have been shown to be TbA₃·HA, TbA₃·2DBBP and TbA₃·2TBP. Their extraction equilibrium constants have been calculated with the following results: for Tb³⁺ (10^-5M)/HNO₃,NaNO₃(μ=0.1)/PMBP-C₆H₅Cl system,K_e=[TbA₃·HA]₀[H⁺]³/[Tb³⁺][HA]0⁴=1.17×10^-3(15℃) for Tb³⁺ (10^-5M)/HNO₃,NaNO₃(μ= 0.1)/PMBP-DBBP-C₆H₅Cl system,K_s,c=[TbA₃·2DBBP]₀[H⁺]³/[Tb³⁺][HA]0³[B]0²=2.19×10²(20℃) for Tb³⁺ (10^-5M)/HNO₃,NaNO₃(μ=0.1)/PMBP-TBP-C₆H₅Cl system,K_s,e=[TbA₃·2TBP]₀[H⁺]³/[Tb³⁺][HA]0³[B]0²=1.37×10¹(20℃) The mechanism of the extraction and structures of the extracted species are discussed.