In this study, the hydrocarbon emission from a three-way catalytic converter with a gasoline engine was investigated experimentally. The results reveal that converters have lower catalytic oxidation activity for methane than the other hydrocarbons, and degradation of the catalyst with use results in a significant effect on methane emission. The maximum methane concentration in exhaust using a converter with 78,000 kin use is about 100 ppm, which is 1.4 times that for a new converter. The light-off temperature of the catalytic converter for hydrocarbon oxidation also rises from 548 to -598 K with an increase;e in the operating age of catalytic converter from 0 to 78,000 km. The ratio of CH4, concentration to total hydrocarbon increases with increasing vehicle speed and methane emission from the fuel of regular grade is, higher than that of the premium grade. (C) 2004 Elsevier Ltd. All rights reserved.