The oxygen reduction reaction (ORR) was studied using a rotating disk electrode. The kinetic current density (cd) at 0.7 V-RHE was reduced by more than a factor of 10 on pre-reduced Pt and a factor of 2.5 on pre-oxidised Pt when comparing ORR rate for pure sulphuric acid to that in 0.18 M H2SO4 + 0.1 M NH3 solution which was the highest ammonium concentration studied. The Tafel slope at low cd on pre-reduced Pt is about -90 mV/decade in the presence of ammonium compared to about -74 mV/decade in Pure acidic solution. The Tafel slopes at high cd is not affected by ammonium. The Tafel slope of ORR on pre-oxidised Pt surface is not significantly affected by ammonium. The exchange cd is lower in the presence of ammonium both for pre-reduced and pre-oxidised Pt. The direct origin of the loss of ORR activity has not been firmly resolved. Adsorption and oxidation of ammonium including intermediate species as well as nitrogen-oxygen species formed at high potentials may be involved. (c) 2005 Elsevier Ltd. All rights reserved.