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Monolithic oxidation converters can become plugged with manganese oxide deposits when the gasoline contains the antiknock additive MMT. Engine dynamometer studies showed that the rate of converter plugging depended on the catalyst inlet temperature and the concentration of MMT in the fuel. Converter plugging was not affected by base fuel or engine oil composition, and it occurred as readily with a bare monolithic support containing no catalytic material as with a production catalyst. Monolithic converter plugging appears to be a physical, rather than a chemical phenomenon, in which the manganese oxide collects primarily on the inlet edge of the converter.