A selective NOx reduction in lean conditions on Rh-based catalysts using CO and hydrogen as reductants was investigated. Over specified Rh-based catalysts, NOx has been found to be selectively reduced with CO as a main reductant at a high efficiency when hydrogen is concomitant as an assistant reductant. Furthermore, the presence of SOx in reaction gases drastically enhances this selective NOx reduction and the activity increases with time, followed by saturation of the NOx reduction. A reaction mechanism of this NOx reduction is proposed based on results of in situ FTIR. In addition, the highly active Rh supported on a beta-zeolite is characterized using TEM, XRD and XPS. (c) 2008 Elsevier B.V. All rights reserved.