Adsorption of NO on Co-ZSM-5 leads to formation of dinitrosyls at 1895 cm~1 and Co2`(NO) 2 [l s (NO) at 1812 cm~1], Co3`ÈNO linear species [l(NO) at 1950 and 1937 cm~1] and NO`occupying cationic l as (NO) zeolite positions [l(NO) at 2133 cm~1]. The NO`and Co3`ÈNO species are of low stability and can be removed by evacuation at ambient temperature. The species start to decompose at 200 ¡C, thus Co2`(NO) 2 forming Co2`ÈNO compounds [l(NO) at 1857 cm~1]. The latter are stable up to 350 ¡C. Water strongly suppresses the formation of NO`and Co3`ÈNO. Its e †ect on the dinitrosyls is, however, weaker Co2`(NO) 2 and even small amounts of water favour the formation of species. The dinitrosyls of Co2`do not Co2`(NO) 2 react with oxygen at ambient temperature but are oxidized in an atmosphere above 100 ¡C. However, they O 2 react easily with a mixture forming surface nitrates as Ðnal products. This process is almost NO ] O 2 una †ected by water. The species start to interact with ethane at 100 ¡C and water has been Co2`(NO) 2 detected as a reaction product. These results account for the (although low) activity of Co-ZSM-5 in the reduction of NO in the absence of oxygen. The only stable species formed in a atmosphere are NO ] O 2 di †erent kinds of surface nitrates (observed in the 1650È1500 cm~1 region). The latter start to interact with ethane at 100 ¡C and nitriles are suggested as interaction intermediates. The role of di †erent surface species and the e †ect of the reactants and reaction products on the SCR of NO over Co-ZSM-5 are discussed.

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