The reaction mechanism of selective catalytic reduction of NOx with decane on acid and iron-exchanged MFI-type zeolite was investigated by operando FTIR spectroscopy and a special reactor cell enabling the use of water-bearing gas mixtures. Water has a dramatic influence on the reaction pathway. The formation of organic nitro and nitrite compounds does not proceed via chemisorbed states of NOx as observed in SCR with dry gases. (c) 2005 Elsevier Inc. All rights reserved.