The surface compositions of supported Pt50Rh50 and Pd50Rh50 three-way catalysts (TWC) have been simulated by Monte-Carlo (MC) technique and used for calculating the activity of the catalysts for CO oxidation and NO reduction. Pd-Rh shows a higher activity than Pt-Rh for CO oxidation in presence of O-2, while Pt-Rh is a better CO oxidation catalyst in absence of 0,. For NO reduction, Pt-Rh nanoparticles are better catalysts both in presence and absence of O-2. beta-N-2 is found to be the most dominant channel for N-2 formation on Pt-Rh nanoparticles, while, delta-N-2 is found to be the most dominant pathway for N-2 production on Pd-Rh. The poisoning effect of sulphur is found to be more on Pd-Rh catalyst than on Pt-Rh catalyst. (C) 2004 Elsevier B.V. All rights reserved.