Influence of sulphur on the catalytic activity of supported Pt50Rh50 nanoparticles for the CO + NO + O-2 reaction has been studied through a microkinetic model. The turnover number Of CO2 molecules per surface atom per second has been found, as expected, to decrease with increase in sulphur coverage. For the CO + NO reaction, the turnover number is smaller by 2-4 order of magnitude compared to the CO + O-2 and CO + NO + O-2 reactions. Irrespective of the sulphur coverage on the surface, the turnover numbers of CO2 molecules are found to be much higher than the turnover numbers of N-2 molecules. Furthermore, the turnover number of N-2 molecules for the CO + NO and CO + NO + O-2 reactions are found to be identical under identical kinetic conditions. (C) 2003 Elsevier B.V. All rights reserved.