The kinetics of the ligand-exchange reactions between Ceedta and dtpa and between Tbedta and dtpa were studied spectrophotometrically at 25° in the pH range 4.4-6.5. In the Tbedta-dtpa system, the exchange takes place by H+-catalyzed dissociation of the complexes and also by collision of the complexes and the dtpa ligand (dtpa5-, Hdtpa4- and H2dtpa3-). In the Ceedta-dtpa system, attack by the dtpa ligands is very effective, and thus the reaction occurs predominantly via an associative mechanism. The reaction rate increase accompanying elevation of the pH can be interpreted by the higher efficiency of collision with dtpa species containgng less protons. In the reaction of Ceedta, formation of a very rapidly produced intermediate was detected, in which, besides the edta, one of the imda groups of the dtpa is also coordinated. The rate-determining step of the reactions is the further transformation of this mixed ligand complex intermediate, the dissociation of edta from the complex.