This work describes a batch single-stage electrochemical process to remove quantitatively the ammonium ion from produced waters from petroleum exploration of the Campos' Basin, seeking to fulfil the directories of the National Brazilian Environmental Council. The anode was made out of titanium covered by a layer of RuO2 + TiO2 oxides (Dimensionally Stable Anode), whereas the cathode was made out of pure titanium. Anodic and cathodic compartments were separated by a membrane. The applied current varied from 0.3 to 1.5 A. As the current increased NH4+ removal was faster and pH was rapidly decreased to 3. The pH of the anodic compartment increased to similar to 10. When the current was 0.92 A chlorine evolution was observed after 40 min or only 15 min when that current was 1.50 A. In this voltage a deposit containing alkali-earth metal hydroxides/sulphates was formed on the membrane surface of the cathode side, thus suggesting a diffusion process from the anodic to the cathodic compartment. The maximum current applied to the cell must not exceed similar to 0.70 A in order to avoid chlorine evolution. Ammonia removal was over 99.9 wt% at 0.68 A in about 75 min. (C) 2008 Elsevier B.V. All rights reserved.