In the present work, the mechanisms involved in NO-char heterogeneous reduction have been studied using a synthetic coal char (SC char) as carbon source. Another synthetic char (SN char) without nitrogen in its composition has also been employed in these studies. Isothermal reduction tests at different temperatures have been carried out. Two temperature regimes were considered: low temperature (T < 250 degrees C) where NO chemisorption takes place and high temperature (T > 250 degrees C) where NO-C reaction occurs. Step response experiments combining consecutive reaction stages with NO and (NO)-N-15 were performed in order to determine the role of nitrogen surface complexes, C(N), in the reduction process. The results revealed N-2 and CO2 to be the main reduction products under the experimental conditions employed in this work. NO chemisorption at lower temperatures results in N-2 emission and surface complexes (mainly oxygenated) formation, while char gasification by NO involves a direct NO attack on the char surface to form surface complexes. As a consequence of desorption of these complexes new sites of reaction are created. (c) 2006 Elsevier Ltd. All rights reserved.