Reactivity of the NO and NO2 adspecies in the coadsorption of NO with CH4 and O-2, and the effect of Si/Al ratio of In-ZSM-5 were studied by FTIR in situ. The relation between the adsorbed species and catalytic activity in the SCR of NOx to N-2 was also investigated. The adsorption of NO over this catalyst was performed at room temperature with pure NO followed by purging with vacuum. When NO was introduced to the samples, three peaks were observed by FTIR: 1622 and 1575 cm(-1), which can be assigned to adsorbed (ONO)- over InO+ site and NO2 over InO+ site, respectively, and at 1680 cm(-1) corresponding to NO3--H. Coadsorption of nitrogen monoxide, methane and oxygen at room temperature of the samples with Si/Al ratio of 17( a), 27(b) and 50(c), allowed us to determine that sample (b) has large amount of NO2-InO+ adsorbed species, which are the most important intermediates in the SCR of NOx. The bands at 1575 and 1680 cm(-1) are more intense in samples ( a) and ( c). When the coadsorption of the mixture was performed at 400 degreesC, we can see that the adsorbed species are larger in sample ( b). Taking into account the catalytic performance of the catalysts and the PAC results obtained by us earlier, this last indium specie, only present in the sample with Si/Al = 27, should be associated with the catalytic active specie for the SCR of NOx.