Negatively charged copper and cadmium ferricyanide sols have been perpared in a fairly pure condition with a view to deteremine their true zeta potential, in the presence of equicoagulating concentrations of different electrolytes. In the zone of rapid coagulation, this has been found to be 16.4 m.v., for copper ferricyanide sol. For cadmium ferricyanide sol, this has been found to be 24.7 m.v. in the zone of slow coagulation.