Contamination of NaOH with chlorate constitutes a major problem for the chlorine-alkali industry, particularly when electrolytic cells based on the diaphragm process are employed. In this paper, pilot and laboratory cell experiments revealed that chlorate contamination in diaphragm cells also inhibits hydrogen evolution and gives rise to a significant increase in electrical energy consumption. Electrolysis carried out under conditions that simulated the industrial process (current density 240 mA cm(-2); temperature 90 degrees C; brine flux 23 L cm(-2) h(-1)) revealed that chlorate formation depends on brine flux and NaOH production. The inhibitory effect of chlorate on the main cathodic reaction was demonstrated in bench cell experiments, with cathodic displacement of the hydrogen evolution reaction by more than 100 mV in the presence of 0.4% chlorate compared with ideal conditions in which chlorate formation was absent. This hydrogen generation overpotential can charge the total electric energy balance in more than 5% of the total value, consisting of a critical loss for this process. (C) 2010 Elsevier Ltd. All rights reserved.