As a response to experiences of in-service plugging of police vehicles, the Automotive Emissions Laboratory has examined 20 high-mileage cars from the New York State Thruway Authority, whose total fleet numbers approximately 100 vehicles. Fourteen catalysts from twelve vehicles were found to be at least 25 percent physically plugged with thin deposits distributed on the front surface of the forward monolith. Analysis of these catalyst deposits by X-Ray Fluorescence indicated that the plugging material was composed mainly of oil additive components (zinc, calcium, barium, phosphorous) as well as iron and lead. Except for one converter with a deposit assay of 25 weight percent manganese, these deposits were in the range of 0.2 to 9.2 weight percent manganese.Our results indicate that monolith catalysts can plug in the absence of significant levels of manganese. However, increased manganese levels in gasoline result in higher levels of manganese aerosols in exhaust streams which may combine with oil additive components to accelerate the plugging process.