A study of the influence of sodium ions on the solid polymer electrolyte (SPE) water electrolysis is reported. Different poisoning modes (anode poisoning and cathode poisoning) and different cathode catalysts (Pt/C, Pd/C and RuO2) were compared. The results showed that the anode poisoning had more severe effects than cathode poisoning. The cell voltages increased by 0.730 V and 0.250 V respectively. In the anode poisoning, the cathode potential descended dramatically by 0.570 V. Meanwhile, the pH of cathode feed water increased to above 11.0 due to the reaction of 2H(2)O + 2e(-) = H-2 + 20OH(-) taking place. However, the cell voltage increased mainly caused by the anode potential in the cathode poisoning. The results also showed that the poisoning results were similar for different cathode catalysts. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.