Mixed potential ammonia exhaust gas sensors provide a high capability for applications in harsh environments when appropriate means are conducted to stabilize the electrodes catalytic activity. The discussed sensor utilizes oxygen ion conducting yttria stabilized zirconia and gold electrodes, one of which is covered with an SCR active film to establish ammonia sensitivity and selectivity. The used SCR catalyst is well-known and developed especially for ammonia SCR exhaust gas aftertreatment systems. Electrochemical half-cell measurements vs. a Pt reference indicate that both electrode configurations are electro catalytically active for ammonia oxidation, but the activity is more pronounced at the additional SCR catalyst layer. The results of the half-cell tests including ammonia dependent voltage and oxygen interference agree very well with the performance of the ammonia sensor device. The behavior of the catalyst electrode cover is characterized separately by activity measurements of powders. The sensing mechanism combines electrochemical reactions at the three phase boundary and chemical reactions at the catalyst material. (C) 2010 Elsevier B.V. All rights reserved.