Zeolite Y was prepared by a dense-gel method and subsequently modified either by treatment with diethylamine (DEA), sodium hydroxide (NaOH) or disodium ethylenediaminetetraacetate (Na 2 H 2 EDTA), or by sequential treatment (H 4 EDTA-NaOH or H 4 EDTA-NaOH-Na 2 H 2 EDTA). Individual treatment did not succeed in introducing mesoporosity in the parent zeolite Y, while the sequential treatment lead to formation of mesopores. After introduction of Cu 2+ ions, the obtained materials were studied as catalysts for the selective catalytic reduction of NO x by NH 3 (NH 3 -SCR, DeNO x ). The catalytic investigations reveal a similar NO conversion for all Cu-containing catalysts up to 450 °C, independent of the introduced mesoporosity. Insights into the dynamics of NH 3 -SCR intermediates through rapid scan FT-IR show that for Cu–Y, the rate-determining step is the formation of the mixed [Cu(O − )(NH 3 ) n -1 (NO)] 2+ complexes, which initiate the NH 3 -SCR reaction. • Formation of mesopores in zeolite Y by sequential treatment. • NO conversion for Cu–Y independent of the introduced mesoporosity. • Formation of the mixed [Cu(O − )(NH 3 ) n -1 (NO)] 2+ complexes, initiating NH 3 -SCR.