A series of in-use catalysts having mileage of 22,000 to 43,000 miles was characterized to determine the effect of the fuel additive MMT. The analytical techniques included visual examination, x-ray fluorescence, x-ray diffraction, optical microscopy, scanning electron microscopy, and electron raicroprobe. In addition, catalyst activity was measured and compared to the catalyst activity from a pulsator aged catalyst without the MMT additive in the feed gas composition. Characterization results show a significantly thick layer (5-20 microns) covering the surface of the catalysts which results in the increase of mass transfer resistance. Steady state R and light-off measurements indicated catalyst efficiency is also significantly reduced as exposure to MMT is increased.