The deterioration of three way catalysts for the reduction of NOx from the exhaust of natural gasfueled engine cogeneration systems was studied using the analyses of used catalysts and accelerated tests of trial catalysts which contain a variety of species and amounts of noble metals. The results show that the catalysts for gasoline-fueled automobiles do not have sufficient activity of methane oxidation after a couple of thousands hours operation, the window in which NOx and CO are simultaneously converted shifts to a rich air/fuel (A/F) ratio, though the window still exists. This means that NOx is not removed at the controlled A/F ratio. The results of the tests using artificially deteriorated catalysts clarified that the deterioration is caused by the sintering of noble metals. Further, from the experiments varying species and amounts of noble metals, it was also proven that Pt plays an essential role in methane oxidation and that the shift of the window can be suppressed by maintaining the surface area of Pt with increases in Pt amounts and in the Pt/Rh ratio. Besides, the increase in Pt amount contributed to an increase in the oxygen storage capacity of the catalyst due to CeO2 and enabled operation under dynamic A/F control for a long time.