The dissolution of ThO2 powdered samples was examined in various conditions of pH and concentration of anions in the leachate. The first part of this paper describes the influence of pH on the dissolution of ThO2 in both nitric and hydrochloric media. The partial order relative to the proton concentration and the apparent normalized dissolution rate constants were determined. The second part of the paper describes the influence of other ligands such as perchlorate, chloride, sulfate, and hydrogen peroxide on the dissolution kinetics (at pH 1). An increase of R-L was observed correlatively with the increase of complexing affinity of the ligand with Th. While nitric and hydrochloric media, which are weakly complexing, lead to R-L values with the same order of magnitude as those for perchlorate media, the presence of sulfate or peroxide in the leachate significantly enhances the dissolution of ThO2. Consequently, the dissolution mechanism can be explained by the weakening of Th-O bonds through the formation of surface complexes at the solid/liquid interface, which enhance the detachment and thus accelerate the global dissolution. In addition, the dissolution of Th0.87Pu0.13O2 solid solution was also examined. The increase of the dissolution kinetics of Th0.87Pu0.13O2, in comparison with that of ThO2, is considered to be caused by the presence of hydrogen peroxide formed by radiolysis of the leachate. Moreover, the redox properties of plutonium in acidic media, like disproportionation of Pu(IV) and Pu(V) and reduction of Pu(VI) and Pu(IV) in Pu(III) by H2O2, probably increase the dissolution of plutonium.