The ammonia decomposition reaction over molybdenum-based catalysts is an example for the complex influence of different factors, such as phase composition; size of crystalline domains, or defect concentration, on the catalytic behavior of a material. In situ powder diffraction allows the direct analysis of how catalysts change during a reaction with respect to the atomic structure or microstructure in terms of defects or size changes. In this article, the influence of catalyst treatment such as pre-reduction or ball milling on the catalytic properties is discussed in detail. (C) 2013 Elsevier Inc. All rights reserved.