NOx trapping capability of "NOx storage-reduction" commercial catalysts (4-9 wt% Ba-containing three-way catalysts) was compared to that of bulk barium carbonate and alumina-supported barium carbonate from Rhodia (9 wt% Ba). These samples were characterized by infrared spectroscopy, X-ray diffraction, HRTEM and EDX. It was shown that bulk barium carbonate was partially converted to barium nitrate in flowing NO/O-2 mixture without CO2. Thermodynamic calculation showed that bulk barium nitrate could not form in the presence of CO2-containing gas exhausts. Using HRTEM and EDX, it was evidenced that barium was engaged as either large barium carbonate crystals or highly dispersed barium species on the alumina support. NOx storage experiments using gas mixtures containing or not O-2 or CO2, confirmed firstly that NO was stored on barium trap only via NO2 and secondly that NO2 and CO2 are competing for the same barium trapping sites. The fact that no significant amount of stored NOx could be evidenced in the bulk barium carbonate, suggested that over the catalytic surface, the well dispersed barium phase can play an important role in the NOx trapping properties of these catalysts.