The hydrolysis constants of Th(IV) were determined using the solvent extraction method in a NaClO(4) Solution at I = 0.1 and 25 degrees C. A trace amount of thorium below the solubility limit was used in order to avoid the formation of polynuclear species and colloids in the experiment. The values were compared with the literature data. Considering the effect of competitive hydrolysis reactions under weak acid conditions, the complexation constants of Th(IV) with a series of dicarboxylic acids containing different alkyl chain lengths were determined.