Studies have been made of the electrophoretic behavior of various samples of plutonia and thoria. Significant differences were observed between samples of the same compound having a different history, but general common trends were apparent. Differences in behavior between plutonia and thoria were no greater than those between different samples of either compound.Adsorption isotherms were calculated using a standard model of the double layer. The data suggest that both plutonia and thoria surfaces adsorb multivalent cations much more readily than monovalent ones. Anions are adsorbed to a varying degree, with the notable exception of nitrate. Both hydrogen and hydroxyl ions significantly affect the surface charge density. The adsorption isotherms do not obey a simple Langmuir law.