The driving force of electrodialysis is applied voltage and electric current. In the event that the module is composed of a number of mutually hydraulically connected subunits (membrane pairs), electric current passes through a part outside the active part along peripherals providing the supply of raw materials. This work focuses on the measurement of these currents that are lost inefficiently. The influence of module geometry on electrodialysis, conductivity of working solutions and applied voltage were studied. The influence of the presence of an ion-conductive membrane in the sealing area on the performance of a membrane stack has been proven. A relation between the conductivity of a membrane and the conductivity of electrolytes to the amount of leakage currents has been found. On the contrary, voltage applied does not primarily influence the distribution of leakage currents but it affects the degree of desalination and the conductivity of solutions in collecting and feeding channels. In the pilot and operating modules with 25 and 50 membrane pairs that were used, the calculated leakage current reaches 0.2-1.2% of the total current load depending on the design and operating conditions. (C) 2017 The Electrochemical Society. All rights reserved.