A zirconium oxynitride catalyst was used for the decomposition of ammonia to hydrogen and nitrogen. The onset of catalytic activity at similar to 550 degrees C coincided with the onset of nitrogen ion mobility in the material and a phase change from the initial ss' phase (similar to Zr7O11N2) to the nitrogen-rich ss" ZrON phase (similar to Zr7O9,5N3). No hydrazine formation during an extended time on stream was detectable. Moreover, the onset of activity was also correlated to a rapid change in the electronic structure of the surface accompanying formation of the more active ss" ZrON phase. The results presented here show for the first time a direct correlation among the onset of ion conductivity as a bulk property, a modified electronic structure of the surface, and the catalytic performance of a heterogeneous catalyst. (c) 2007 Elsevier Inc. All rights reserved.