Abstract: In the determination of the stability constants of the TTHA complexes with Am (Ⅲ) and Cm (Ⅲ) by an ion exchange technique (t=25℃,μ=0.5), only the 1:1 complex appears to be formed. The equation used to calculate the stability consatants is:K=D0/D1·H6YT where D1 and D0 are the distribution coefficient of the metal ion with and without the ligand resectively; H6 YT is the total concentration of the ligand present, irrespective of its form; and is equal to H6 YT/Y6-. The stability constants obtained are 1027.44 for Am and 1027.61 for Cm.The difference between the chelating capabilities of DTPA and TTHA with actinides and lanthanides becomes larger with the increase of the number of acetate groups in the amino-polycarboxylic acid.