:Four samples from each of two field-aged catalysts subjected to different field test conditions were investigated. The light-off and conversion performance of each sample was measured in a synthetic exhaust flow reactor system. Time-resolved laser IR spectroscopy was used to investigate the catalyst behaviour under transient conditions.Significant differences in light-off temperatures and transient conversion performance between the samples was observed. The samples taken from the inlet side of the monolith were more deactivated than the corresponding ones from the outlet. However, samples taken from peripheral positions always showed better performance than samples originating from the centre.In order to explain observed variations in activity, the following surface properties were examined: oxygen uptake, specific metal area (CO chemisorption), total surface area (BET) and chemical composition (XPS analysis). It was concluded that thermal deactivation was mainly responsible for the difference between peripheral and central positions while the poisoning due to S, Pb and P had the largest effect on the inlet samples.The total conversion efficiency, as measured by FTP75 tests, was compared to the results from laboratory activity tests and catalyst charactarization data. The mechanisms of ageing and the effect on emissions were correlated to the laboratory results from this study.