A computer model is successfully used to predict emissions from a vehicle, fitted with a catalyst, over a standard driving cycle (the U.S. FTP cycle). Kinetic expressions to describe the catalyst's activity are derived from laboratory measurements. A study showing the effect of changing the precious metal loading of the catalyst on the activity of the catalyst is presented. The impact on model parameters is derived and the model is successfully used to predict the effect of changing precious metal loading on vehicle performance over the FTP cycle. The results indicate the model can be a useful tool in exhaust system design.