The direct decomposition of NO_x at moderate temperatures is a challenge because of the poisoning of the surface of noble metals by oxygen, yet the modification of Pd with Au enables obtaining a highly stable catalyst under an O₂-free feed at 450 °C. Care was taken to ensure that the reaction was not due to the reduction of NO_x with carbonaceous residues by monitoring CO₂ evolution. Pd–Au alloyed domains with a wide range of compositions were evidenced by in situ and ex situ XRD, the Pd content of which decreased with the increasing oxidising nature of the feed due to phase segregation giving rise to PdO and an Au-rich alloy. Under air, the limiting alloy composition was Pd_0.2Au_0.8. The catalyst was yet inactive in the presence of 5% O₂.