The aim of this work was to obtain information about the causes of unusual lack of activation of fresh, unused, 90% platinum-10% rhodium catalyst gauze that was detected after the activation process in an industrial ammonia oxidation plant. To reach the usual efficiency, it was necessary to regenerate the catalyst immediately after the activation procedure by using an acid bath. To avoid future unexpected shutdowns in the ammonia oxidation plant, the possible sources of impurities accumulated on the catalyst surface were investigated by an analysis of the experimental results and technological processes. The absence of accumulation on the unused catalyst surface eliminates the possibility of contamination during the preparation, while the composition of the impurities indicates that it cannot result from the reactor construction material. In mixed feedstock, detectable amounts of the poisons mentioned were not detected either after filtration or before the reactor. All these considerations imply that the time when the catalyst contamination occurred must have been before the contact with the gas mixture. From data obtained, it is hypothesized that the origin of this substance is from the polluted air in the industrial area where the ammonia oxidation plant is located and that the accumulation probably took place during the preparation of the catalyst pad.