In this study. in situ transmission FT-IR spectroscopy was employed to investigate the thermal and selective catalytic reduction of NO, with propylene in nanocrystalline NaY zeolite. NO was the major product in the thermal reduction of NO,. In absence of oxygen and water, selective catalytic reduction of NO2 with propylene at low temperatures (T less than or equal to 473 K) resulted in the complete reduction of NO2 to N-2 and O-2. Isocyanate and nitrile species formed on extra framework alumina sites are found to be important intermediates. Based on kinetic studies, a pathway involving radical reactions appears to be the most plausible mechanism for the complete reduction of NO2. (C) 2004 Elsevier B.V. All rights reserved.