In acetonitrile solutions, the exchange reaction is bimolecular in the Tl++ 18C6 system, while in the Tl++ pentaglyme system the associative-dissociative and the bimolecular mechanisms coexist at room temperature and the bimolecular exchange reaction dominates at 263° K. For the bimolecular mechanism in the case of Tl++ 18C6 and the associative-dissociative mechanism in the case of Tl++ pentaglyme, the activation energies of the exchange reactions change with temperature. At 298° K, in the Tl++ 18C6 system the activation energy for the bimolecular exchange reaction is ≈ 2 kcal.mol−1and exchange rate constant (k1) is (4.1 ± 0.1) × 107s−1mol−1; in the Tl++ pentaglyme system, the activation energy for the associative-dissociative exchange reaction is ≈ 5 kcal mol−1and the decomplexation rate constant (k−2) is (2.2 ± 0.4) X 105s−1. The activation energy for the bimolecular exchange in the Tl++ pentaglyme system was determined to be 3.00 ± 0.05 kcal.mol−1and the exchange rate constant (3.0 ± 0.1) X 108s−1mol−1.