The limitation of the use of partial current blocking for determining partial conductivities in mixed ionic electronic conductors, MIECs, is reviewed. The discussion refers to the Hebb-Wagner polarization method for determining the electronic conductivity and to its analogue for determining the ionic conductivity. These two methods suffer from severe problems that may render results useless because of false interpretation. The problems are discussed. New experimental evidence for some problems is presented. A four point, Hebb-Wagner type, measurement is discussed. It allows one to detect whether the correct experimental conditions prevail.