The present work reports the hydrolysis of ions of the series of lanthanides (III) and actinide (IV) elements in dilute aqueous solutions. It has been systematically examined in the presence of sodium perchlorate, which has been used for maintaining the solution at constant ionic strength. The number, the nature of the species in solution and their hydrolysis constants logb 10i for all ions at I = 0.1 mol dm-3 and at 25°C were determined by different softwares: Superquad and Sirko_P. A pH-potentiometric method was used with glass electrodes to determine the equilibrium constants K i : M n + H 2 O b M(OH) (n−1)+ + H + where M n+ represents Ce 3+ , Pr 3+ , Nd 3+ , Eu 3+ , Sm 3+ and Th 4+ ions. For the lanthanides, hydrolysis increases with the increase of the atomic number and the contraction of the ionic radius. The Th 4+ ion undergoes significant hydrolysis. To cite this article: