Catalysts aged under different on-road conditions were analysed with respect to their conversion of CO and HC at step changes of the synthetic exhaust gas composition. Time resolved diode laser spectroscopy and fast response FID analysis were used to characterise the catalyst response to transient changes of CO and hydrocarbons in the exhaust gas. The oxygen storage capacity was monitored at various conditions; flow rate, catalyst temperature, previous exposure to oxidizing or reducing atmosphere and amplitude of the perturbation.The technique appeared to provide a sensitive probe for analysis of the dynamic oxygen storage capacity of new and aged catalysts at exhaust like conditions. The results correlate well with the transient emission performance during vehicle tests.Further, surface characterization using SEM/EDS and XPS techniques indicated that phosphate formation was the most probable cause of deactivation. A strong relationship between phosphate content and catalyst performance properties was found. The possible nature of this phosphorous deactivation function is discussed.