A manganese fuel additive, MMT, is now being used in many unleaded gasolines to improve their octane quality. Use of MMT at concentrations up to 0.033 g Mn/ℓ (0.125 g Mn/gal) is expected to increase. To determine the effect of MMT on exhaust emission control systems, five cars were tested for 80 000 km (50,000 miles) using a driving schedule which included 113 km/h (70 mph) steady speed driving. In this type of operation, use of MMT caused: plugging of monolithic converters located close to the exhaust manifold; partial plugging of an underfloor bead converter; an increase of hydrocarbon emissions from the engines; and excessive spark plug deposits. However, use of MMT apparently enhanced catalytic converter oxidizing activity and did not substantially affect octane requirement increase. These preliminary data suggest that use of MMT in commercial gasolines may cause problems with exhaust emission control systems now in use.