Keggin heteropolyacids (HPAs) have been known to be efficient NOx absorbers for many years, and to decompose a significant fraction of the NOx species pre-absorbed at 100-150 °C into harmless N2 and O2 upon rapid heating to 450 °C. However, this capability of Keggin HPAs to directly decompose NOx in the absence of reducing agent in the feed has never been studied under more realistic reaction conditions, e.g. under continuous NOx feeding and in the presence of O2 in the feed. This was done for the first time in the present work, over the widely used H3PW12O40 Keggin HPA, at 380 °C. H3PW12O40 was shown to be active in the direct decomposition of continuously fed NOx, with the decomposition process occurring through the involvement of NOH + adspecies. O2 was found to play a dual role. On the one hand, it led to the oxidation of NO into NO2 in the feed, with NO2 deactivating the HPA units. On the other hand, it was found to initiate the diffusion of NOx into the bulk of the HPA crystals, which significantly enhanced the durability of the activity of the HPA in the direct decomposition of these NOx, provided that the diffusion pathway in-between the bulk Keggin units was already accessible thanks to the presence of NOx species pre-absorbed at RT. By providing the key factors involved in the direct decomposition of continuously fed NOx over H3PW12O40, the present work paves the way for future investigations that might focus, for instance, on tuning the chemical composition of the heteropolyanions in order to make the HPA more resistant to deactivation.