The Monte-Carlo simulation technique has been used to investigate the effect of sulphur impurity on the shell composition profile of unsupported Pt-Rh nanocatalysts. For temperatures 400, 700, 1000 and 1300 K, the top shell of clean Pt-Rh nanocatalysts is found to be mildly Pt-enriched, while the second shell is Pt-depleted. The Pt concentration of the top shell shows a maximum at T = 1000 K. In presence of 10% and 5% sulphur impurity, the top shell shows strong Rh enrichment, while all the other shells show Pt enrichment. This is true for all the four temperatures for which the composition profiles have been studied. The extent of Pt fraction in shells depends linearly on sulphur impurity concentration in agreement with experimental results. (C) 2002 Published by Elsevier Science B.V.