In this paper mechanistic aspects involved in the formation of N2O over model Pt-Ba/Al2O3 and Rh-Ba/Al2O3 LNT catalysts are discussed. The reactivity of both gaseous NO and of stored NOx (nitrates) has been studied, with simultaneous surface characterization by operando FT-IR spectroscopy, using different reductants (i.e. H-2, CO, CO + H-2, CO + H2O) both under isothermal conditions and temperature programming. The results show that N2O formation may occur during both the lean/rich and rich/lean switches (primary and secondary N2O, respectively). In particular: i) primary N2O formation involves the presence of gas-phase NO and partially reduced metal sites; ii) N2O formation increases in the presence of CO because the reduction of the metal sites is slower, thus favoring N2O formation upon the lean/rich transition; iii) residual reducing species onto the surface (i.e. NCO-, CO) can react with NO giving the secondary N2O peak. A reaction pathway for N2O formation is suggested where metal sites (Pt or Rh) catalyse the NO dissociation reaction into N- and O-adatoms; N-species further interact with undissociated NO molecules leading to the formation of N2O (primary N2O). In additions, isocyanates formed during the NOx reduction in the presence of CO may participate in the N2O formation upon reaction with NO during the lean phase (secondary N2O). Pt- and Rh-based catalysts show similar behavior even if Rh-based catalyst is less reactive than Pt-based sample likely due to the lower dispersion of the noble metal.