The complex formation between the uranyl (UO22+) ion and the diethylene triaminopentaacetate ligand (DTPA) has been investigated at 25 degrees C, in a 3M sodium perchlorate medium. The overall protonation constants beta(H)(j) of the free ligand have been previously determined in this ionic medium: six protonated species (H(j)A), with j ranging, from 1 to 6, together with the free anion A(s-) have been identified in the concentration range from (3 . 10(-3) to 13 . 10(-3)) mol . kg(-1). Four complex species, H(2)UO(2)A(-), HUO(2)A(2-), UO(2)A(3-), and UO(2)AOH(4-), have been identified in the total uranyl concentration range from (1.1 . 10(-3) to 5.7 . 10(-3)) mol . kg(-1), and their overall stability constants determined, keeping the metal to ligand ratio equal to 1. Owing to the complex species formation, uranyl ions could be kept in solution until -log h = 7.