The effect of the operating parameters current density, feed flow rate, brine and caustic concentrations, and temperature of a laboratory-scale chlor-alkali membrane cell on its overall performance (power consumption) was studied. Electrochemical impedance spectroscopy (EIS) was used to evaluate in situ the ohmic resistances of electrolytes and membrane. The cell temperature and the brine concentration were the most important variables on the cell voltage. The gas bubble evolution had a great effect on the ohmic resistance, especially at the anode side, where the gas void fraction was about 10%.