Abstract: The solution chemistry of Th(Ⅳ) with several hydroxypyridinones, 1-hydroxy-2 (lH) -pyridinone (1,2-HOPO) and 3-hydroxy-2 (1H)-pyridinone (3,2-HOPO) has been studied using potentiometric and spectrophotometric methods. The potentiometric measurements are described for the solution containing different ratios of hydroxypyridinones to Th(Ⅳ) in 0.10mol/1 KNO3 solution at 25℃. Several complexes of Th(Ⅳ) and hydroxypyridinones are formed in the solutions at different systems and pH values. The complexes of Th(l,2-OPO)3 + , Th(l,2-OPO)22+ Th(l,2-OPO)3+ and Th(l ,2-OPO)4 are formed at pH 3.4, 4.0, 4.7 and 5.8, respectively. It is found that similar complexes are also formed for the system of 3, 2-HOPO and Th(Ⅳ), but the buffer region of metal complexation for 1,2-HOPO is lower in pH. The formation constants of the complexes are calculated using a non-lin-ear least square program to refine potentiometric titration data by CDCCYBER 170/825 computer. It is found that the formation constants are as follows. logβ110=9.3(1), logβ130=17.2(4), logβ130 = 23.4(5) and logβ140 = 29.0 (Ⅰ) for Th(Ⅳ)-1, 2-HOPO system and logβ110=10,4(2), logβ120=19.2(2), logβ120 = 26.6 (6) and logβ140 = 32 .4 (5) for Th (Ⅳ)-3, 2-HOPO system. The pM values of 1, 2-HOPO and 3,2-HOPO for Th(Ⅳ)are 14.1(2)and 12.5(8) at pH 7.4, MT = 1μumol/l andLT = 10μmol/l respectively.These results are to be used in the design and refinement of actinide-spe-cific macro-chelate sequestering agents.