The NO (x) adsorption mechanism on Pt/BaO/Al2O3 catalysts was investigated by performing NO (x) storage/reduction cycles, NO2 adsorption and NO + O-2 adsorption on 2%Pt/(x)BaO/Al2O3 (x = 2, 8, and 20 wt%) catalysts. NO (x) uptake profiles on 2%\Pt/20%BaO/Al2O3 at 523 K show complete uptake behavior for almost 5 min, and then the NO (x) level starts gradually increasing with time and it reaches 75% of the inlet NO (x) concentration after 30 min time-on-stream. Although this catalyst shows fairly high NO (x) conversion at 523 K, only similar to 2.4 wt% out of 20 wt% BaO is converted to Ba(NO3)(2). Adsorption studies by using NO2 and NO + O-2 suggest two different NO (x) adsorption mechanisms. The NO2 uptake profile on 2%Pt/20%BaO/Al2O3 shows the absence of a complete NO (x) uptake period at the beginning of adsorption and the overall NO (x) uptake is controlled by the gas-solid equilibrium between NO2 and BaO/Ba(NO3)(2) phase. When we use NO + O-2, complete initial NO (x) uptake occurs and the time it takes to convert similar to 4% of BaO to Ba(NO3)(2) is independent of the NO concentration. These NO (x) uptake characteristics suggest that the NO + O-2 reaction on the surface of Pt particles produces NO2 that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NO (x) uptake, this spill-over process is very fast and so it is able to provide complete NO (x) storage. However, the NO (x) uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to Ba(NO3)(2). The formation of Ba(NO3)(2) around the Pt particles results in the development of a diffusion barrier for NO2, and increases the probability of NO2 desorption and consequently, the beginning of NO (x) slip. As NO (x) uptake by NO2 spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO2 uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase NO2 concentration.