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The accuracy of numerical predictions for a high exhaust gas temperature through a converter or when an extremely active catalyst is modelled has been shown to be in doubt. Here mass transfer effects dominate the overall conversion efficiency of the after-treatment system. The mass transfer calculations are based on the Sherwood number and the concern is that there is a significant range of Sherwood numbers to choose from in the literature. Two significantly differing Sherwood number relationships from the literature have been developed based on different measurement techniques. It was shown that some techniques were unreliable due to the influence of reaction kinetics on the mass transfer data. It was also shown that the use of different Sherwood numbers could lead to up to 55% difference in the predicted cumulative emissions of HC, CO and NOx.