NOx sorption and reduction capacities of 12-tungstophosphoric acid hexahydrate (H3PW12O40.6H(2)O, HPW) were measured under representative alternating conditions of lean and rich exhaust-type gas mixture. Under lean conditions, the sorption of NO is large and is equivalent to 37 mg of NOx/g(HPW). Although a part of these NOx remains unreduced, HPW is able to reduce some of the NOx to produce N-2 by a reaction between the sorbed NO2 and hydrocarbon (HC), but this process is slow. The addition of 1% Pt affects strongly the chemical behaviour occurring during the course of a rich operation. The NO desorption observed at the beginning of the rich phase is strongly accelerated. The direct correlation between NO2 consumption and CO2 production shows that the principal pathway is the reaction CO + NO2 CO2 + NO. In a mixture of reducing gas (CO, HC, H-2), the competition is strongly in favour of CO though in its absence the reaction observed was the hydrogenation of propene to propane.