The study of automotive catalyst deactivation is associated with technical, economic and environmental problems. The deactivation phenomenon has different origins that can be divided into three groups: thermal, chemical and mechanical deactivation. In this work, nine commercial catalysts were analyzed, seven of them aged in an engine bench. A study using scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDS) at different positions of the monolith allowed the probable reasons for catalyst deactivation to be inferred. Thermal deactivation was a common cause for most of the samples. Other mechanisms such as washcoat losses or abrasion, contamination by sulfur and lubricants, and noble metal sintering were also observed for different samples. (c) 2005 Elsevier B.V. All rights reserved.