Simultaneous reduction of nitrates and oxidation of ammonia in aqueous solutions have been carried out in a zero gap solid polymer electrolyte (ZGSPE) reactor, operated galvanostatically in a batch recycle mode. Complete removal of 16.1 mM nitrate and 9.4 mM ammonia was achieved within 45 h with removal rates of 0.057 mol NO3- cm(-2) h(-1) and 0.017 mol NH3 cm(-2) h(-1). Space-time yields of 5.4 kg NO3 m(-3) h(-1) and 0.17 kg NH3 m(-3) h(-1), current efficiencies for nitrogen formation of 24.5% in the nitrate reduction and 1.4% in the ammonia oxidation and energy consumptions of 40.1 kWh (kg NO3-)(-1) were obtained during nitrate reduction, no nitrite was formed and N-2 was the main product under the best conditions. However, an improvement in the selectivity for the ammonia oxidation towards nitrogen is required. Effects of reactant concentrations, temperature and flow rate have been investigated. Use of the ZGSPE reactor could treat a wide range of wastes containing nitrate and ammonia, including those with very low levels of nitrate ions and ammonia. (c) 2005 Elsevier B.V. All rights reserved.