The oxidation of ammonia has been studied using Differential Electrochemical Mass Spectrometry on the three Pt basal planes. The observed products N-2, NO and N2O were obtained for the three platinum single crystals. The sensitivity of the ammonia oxidation towards Pt(100) sites is highlighted as well as the presence of a poison formed during the oxidation that blocks the surface. This species seems to inhibit the N-2 formation at lower potentials than for NO and N2O. (c) 2006 Elsevier B.V. All rights reserved.