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Emission studies were carried out on clear-fueled and MMT®-fueled 100,000-mile Escort vehicles from the Alliance study [SAE 2002-01-2894]. Alliance testing had revealed substantially higher emissions from the MMT-fueled vehicle, and the present study involved swapping the engine cylinder heads, spark plugs, oxygen sensors, and catalysts between the two vehicles to identify the specific components responsible for the emissions increase. Within 90% confidence limits, all of the emissions differences between the MMT- and Clear-vehicles could be accounted for by the selected components. NMHC emission increases were primarily attributed to the effects of the MMT cylinder head and spark plugs on both engine-out and tailpipe emissions. CO emission increases were largely traced to the MMT cylinder head and its effect on tailpipe emissions. NOx emission increases were linked to the MMT catalyst.