Data obtained from studies of the electrophoretic mobility of thoria and plutonia suspensions have been examined in relation to their dispersion and ion adsorption properties. Dispersion data in many cases bear out qualitatively the concept of a critical zeta potential below which flocculation of the suspension takes place, but exceptions to the rule have been found in a number of electrolyte solutions. Adsorption isotherms obtained from tracer studies and direct analysis have been compared with those calculated from electrophoretic data. Differences betweenζandψin the case of hydrogen ions are qualitatively consistent with adsorption on a negative site in the presence of a Stern-type layer of anions. Observations with I−,, Cs+, Sr2+, Ce3+, and Th4+require a qualified interpretation, and reveal the danger of drawing quantitative conclusions from electrophoretic data unsupported by more direct measurements.