NOx adsorption/desorption capacities of barium aluminates and BaSnO3 were measured under representative exhaust gas mixture at temperatures below 550 degrees C and compared to those of bulk BaO. The capacities are high and the test of sorption-desorption is reproducible on barium aluminate and BaSnO3, while this is not the case on BaO. The difference is due to the electronic environment of barium oxide. If BaO is not engaged in a chemical bond, progressive formation of high stability carbonates is observed. This is not the case with barium aluminate and BaSnO3, where carbonation does not take place because the competition between nitrate and carbonate formation is in favour of the nitrate due to its chemical nature. An N-bounded nitrate, with IR frequencies at 1360 and 1415 cm(-1), is formed on barium aluminate and BaSnO3 and not on bulk BaO. (C) 2000 Elsevier Science B.V. All lights reserved.