NOx adsorption/desorption capacities of 12-tungstophosphoric acid hexahydrate (HPW) were measured under representative exhaust lean gas mixture conditions. The amounts of NOx adsorbed were large and equal to 38 mg/g of HPW. On such material desorption is observed during the cooling phase under wet air conditions. This type of behaviour is very unusual and has never been reported previously. NO and NO2 are absorbed and desorbed in an equimolar ratio without nitrate formation. The mechanism of adsorption proceeds by the substitution of lattice water molecules and formation of a [H+(NO2-, NO+)] complex. During the cooling phase and in the presence of water, around 100 degreesC, reverse substitution occurs. (C) 2001 Academic Press.