Abstract: The standard solutions of Ce(SO4)2 and (NH4)2Fe(SO4)2 were standardized simultaneously by potentiometric titration. Sodium oxalate was used as primary standard. In order to avoid the interference of mass exchange between the salt bridge and the measured solution with the result of titration, the titration device was modified. An ion-exchange membrane was set between the salt bridge and measured solution to depress the mass exchange at the interface. It was found that high precision was attained by the present method, giving values of relative deviation between the determined values of the 2 standard solution of 0.55%.