The surface charge behavior of uranium(IV) oxide and thorium(IV) oxide was investigated by potentiometric titration, and the pH point of zero charge (pzc) was determined for both oxides. Crystalline structures were investigated by X-ray powder diffraction, which showed that both oxides were of the cubic system (Fm3m) with a slightly differing unit cell size. The pzc was determined for the oxides at two ionic strengths: 0.01 and 0.1 M (NaClO4). The pzc of UO2 was found to be pH 5-5.5 at both ionic strengths, which was supported by measurement of the isoelectric point. The corresponding result for ThO2 was approximately pH 8, seemingly varying slightly with the ionic strength. The UO2 surface was sensitive to further oxidation, which affects titration experiments by yielding ostensibly high pzc unless the oxidation is prevented. Inert, or at least O-2 and CO2 free, conditions are thus necessary during work with UO2.