Automotive three-way catalytic converters are routinely applied today in the elimination of carbon monoxide, urburned or partially burned hydrocarbons and nitrogen oxides from gasoline engine exhaust gas. Catalyst activity in this case is only indirectly assessed by means of the measurement of overall emissions during a legislated driving cycle. The specific structure of the driving cycles embodied in a legislated emissions test procedure, significantly affects catalyst performance. In addition, catalyst behaviour is affected by the levels of raw engine emissions and catalyst inlet temperatures. The latter are strongly influenced by exhaust system design. The above explain easily the difficulties arising when one wants to discriminate between the behaviour of different catalysts in a specific test procedure. The advent of reliable mathematical models of three-way catalytic converter operation allows for a detailed analysis of catalyst performance over the full legislated cycle, which gives more insight into the catalytic activity itself. In this paper, a specific experimental and computational case study is employed to demonstrate the role of mathematical modelling in the detailed analysis of aged catalyst performance during the US and EU legislated test procedures. Copyright (C) 2000 John Wiley & Sons, Ltd.