Catalytic exhaust gas aftertreatment devices fitted to combustion engines are susceptible to partial deactivation as their operating time progresses. This includes selective catalytic reduction (SCR) reactors meant for NOx emission control. There are several known deactivation mechanisms of SCR reactors already analyzed in detail in the literature. This paper, however, approaches the analysis of reactor deactivation by comparison of exhaust gas characteristics over repeatable cycle for fresh and aged samples of a SCR reactor for non-road mobile machinery. The research aims to outline which parameters describing the SCR reactor’s performance are most affected by its ageing. In order to do that, fresh and aged samples of the reactor were tested under the Non Road Steady Cycle. The acquired emission results, including concentration traces of particular compounds were analyzed. The specific NOx emission of the aged reactor was significantly higher than that of the fresh one. The NOx conversion efficiency of both reactors was found similar at periods of steady engine operation. It was recognized that during transient conditions the NOx conversion efficiency of the aged reactor was decreased. It was found that the main factor contributing to that phenomenon is the drop in the ammonia storage capacity of the aged SCR sample.