The catalytic activity of the pre(p)- and the underfloor(u) three-way catalysts (TWCs), of a bifuel passenger car (compressed natural gas (CNG), gasoline), has been examined at a series of mileage milestones. Over a rather short lifetime of 35,000 km, a significant increase of the unburned total hydrocarbons (THC) has been determined during CNG operation. No comparable THC increase in the exhaust was found during gasoline operation. The THC increase was detected at exhaust temperatures well over 633 K, excluding methane specific light-off deterioration. Possible explanations for this fuel selective behaviour could be identified by the application of different surface techniques. XPS analysis hints to the formation of a metallic Pd layer formed over the catalytically active PdO core, probably having the highest impact on the decreased methane oxidation during CNG operation. Typical lubricating oil based contaminants like P, Ca, and Mg, have been detected on the catalysts' surface as well. (c) 2008 Elsevier B.V. All rights reserved.