Deactivation of a Pt/g-alumina catalyst used for low-temp. ammonia oxidn. was studied using positron-emission profiling (PEP). Evidence is presented that irreversibly adsorbed nitrogen and oxygen species deactivate the catalyst. This deactivation is further accelerated by preadsorption of oxygen on the catalyst. Ammonia strongly interacts with the alumina support. A steady state of the ammonia oxidn. reaction is reached after the alumina sites are satd. with ammonia. PEP expts. show that spillover of oxygen to the support is not significant during the ammonia oxidn.