Extensive vehicle fleet testing has demonstrated that use of MMT can reduce net tailpipe out emissions. The use of fuel containing the octane-enhancing, emission-reducing fuel additive leads to manganese oxide deposits in the vehicle exhaust system. Studies of the physical and chemical effects of manganese oxide deposits on the performance of catalytic converters conclusively demonstrated that MMT does not adversely affect catalytic converters and, in fact, protected the converters from phosphorus and zinc.Despite the overwhelming evidence that MMT is compatible with catalytic converters and vehicle emission control systems, concerns have recently been raised about the effect of manganese oxides on OBD-II catalytic converter monitoring. These concerns are unfounded based on results from studies on the effect of MMT on the dual EGO catalytic converter monitoring system using vehicles and components from an extensive vehicle fleet test program designed in consultation with the EPA and automobile industry. In the study, paired vehicles which accumulated over 100,000 miles on fuel with or without MMT were used. The paper includes results from component testing on vehicles and bench testing. The results of this testing show that MMT does not adversely affect the monitoring quality of the dual EGO sensor method for catalyst monitoring.