tThe addition of minute amounts of H2to the selective catalytic reduction of nitrogen oxides (NOx)by hydrocarbons (HC-SCR) has been shown to promote the reduction of NOxat remarkably lowerreaction temperatures on Ag/Al2O3catalysts. Despite extensive investigations, the origin of this remark-able hydrogen effect is still being debated in the literature. To provide further insights into this, thedesorption-reactivity of nitrates preadsorbed on a Ag (1.6 wt%)/Al2O3catalyst is reported for the first timeunder desorbing feeds of increasing complexity. The results of these transient experiments together withthose of steady-state experiments under selected gas compositions confirm some of the numerous rolesof H2suggested previously such as being responsible for (i) changes in the distribution of Ag species, (ii)promoted HC activation and (iii) lower nitrates coverage because of their lower stability in the presenceof H2. In particular, it was found that nitrates decompose at temperatures as low as 120◦C in the presenceof H2in the desorbing feed, instead of around 300◦C in the absence of H2. The transient experiments inwhich the reactivity of the nitrates was studied under desorbing feeds containing propene (C3H6) as areductant, allowed for the monitoring of the formation-decomposition of organo-NOxspecies (R-NOx).From these experiments, it can therefore be identified that another potential role of H2is to promote theformation-decomposition-reaction of organo-NOxspecies at lower temperatures coinciding with thetemperature of the onset of detection of N2under both transient and steady-state conditions, in line withthe "chemical effect" of H2suggested earlier.