Selective catalytic reduction of NOx (SCR-NOx) with decane, and for comparison with propane and propene over Cu-ZSM-5 zeolite (Cu/ Al 0,49, Si/Al 13.2) was investigated under presence and absence of water vapor. Decane behaves in SCR-NOx like propene, i.e. the Cu-zeolite activity increased under increasing concentration of water vapor, as demonstrated by a shift of the NOx-N-2 conversion to lower temperatures, in contrast to propane, where the NOx-N-2 Conversion is highly suppressed. In situ FTIR spectra of sorbed intermediates revealed similar spectral features for C10H22- and C3H6-SCR-NOx, where -CHx, R-NO2, -NO3-, Cu+-CO, -CN, -NCO and -NH species were found. On contrary, with propane -CHx, R-NO2, NO3-, Cu+-CO represented prevailing species. A comparison of the in situ FTIR spectra (T-O-T and intermediate vibrations) recorded at pulses of propene and propane, moreover, under presence and absence of water vapor in the reaction mixture, revealed that the Cu2+-CU+ redox cycle operates with the C3H6-SCR-NOx reactions in both presence/absence of water vapor, while with C3H8-SCR-NOx, the redox cycle is suppressed by water vapor. It is concluded that decane cracks to low-chain olefins and paraffins, the former ones, more reactive, preferably take part in SCR-NOx. It is concluded that formation of olefinic compounds at C10H22-SCR-NOx is decisive for the high activity in the presence of water vapor, while water molecule's block propane activation. The increase in NOx-N-2 conversion due to water vapor in C10H22-SCR-NOx should be connected with the increased reactivity of intermediates. These are suggested to pass from R-NOx -> CN -> NCO -> NH3; the latter reacts with another activated NOx molecule to molecular nitrogen. The positive effect of water vapor on the NOx-N-2 conversion is attributed to increased hydrolysis of -NCO intermediates. (c) 2005 Elsevier B.V. All rights reserved.