The gas components CO, CO2, HC, NOx and the AFR in the exhaust from a SI engine, both upstream and down-stream of a Pd/Rh catalytic converter, have been monitored using fast response analyzers. Regular sequential step changes in the upstream air/fuel ratio (AFR), between two pre-set levels, have been implemented with both long and short periods between the steps. For transitions from rich to lean conditions, and vice-versa, several distinct zones for the output emissions characteristics, corresponding to different states of the catalyst surface, have been identified. These results suggest that, under reducing conditions, hydrogen is stored on the catalyst surface whereas under oxidizing conditions oxygen is stored by two different processes. These chemical insights facilitate the development of realistic models for tailpipe emissions from engines which are perturbed from steady state running.