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.