A Pt-Rh three-way catalyst used in natural gas-fuelled engine systems for 21 000 h was investigated employing chemical analyses, EPMA, surface area measurements, catalytic activity tests, as well as tests using model poisoned catalysts. Specific deactivation characteristics, such as the decrease in the selectivity of nitric oxide reduction, which cannot be reproduced by heat treatment nor explained by physical poisoning such as blockage of micropores, were observed on the used catalyst. P, Ca, Zn, Fe and trace amounts of Pb were detected as elements which increased in the used catalyst, and Zn, Fe and Pb were observed in the wash coat layer of the catalyst, while P and Ca existed on the surface of the wash coat. Of the model Zn-, Fe-, and Pb-poisoned catalysts prepared by dipping heat-treated catalysts into an aqueous solution of metal salt, only model Pb-poisoned catalyst showed a considerable decrease in the selectivity of nitric oxide reduction. Further, all the characteristics of the deactivation on the used catalyst were qualitatively reproduced by the measurement of the activity of the model Pb-poisoned catalyst at high space velocity. From these results, it is concluded that the selectivity of nitric oxide reduction is decreased by chemical poisoning due to trace amounts of Pb on the used catalyst, and that the absolute rates of nitric oxide reduction as well as the other reactions are additionally reduced by an decrease in the effective surface area of the catalyst due to the covering the surface of the wash coat layer by accumulated compounds consisting of P, Ca, Zn and Fe. Therefore, trace amounts of Pb as well as ash derived from oil additives become an issue even in natural gas-fuelled engine systems when a Pt-Rh three-way catalyst is used for more than several ten thousand hours.