Manganese oxide deposits which are exclusively in the form of Mn3O4, a benign form of manganese, are introduced in the exhaust stream from use of MMT, an octane-enhancing, emission-reducing fuel additive. The physical and chemical effect of these deposits on catalytic converters has generated some controversy in the literature.In this paper, we will focus on the effects that manganese oxide deposits have on catalytic converters. The physical effect of these deposits on the morphology of the converters was investigated by B.E.T surface area measurements, scanning electron microscopy (SEM), and x-ray fluorescence (XRF). The chemical effect was investigated with tests using both slave-engine dynamometers and a pulse-flame combustor to probe for differences in catalyst performance.Data from an extensive vehicle fleet which was tested according to a program designed in consultation with the EPA and the automobile industry will be presented. The automobiles were paired so that one was assigned to clear fuel and its partner to the same fuel with the HiTEC 3000 additive. These automobiles were equipped with three-way catalytic converters with Pt/Rh as the active metals. Each vehicle operated over 75,000 miles, with FTP tests carried out at intervals of 5000 miles. Emissions data from this fleet have previously been presented. Additionally, six 1911 Ford Escorts were similarly tested to 24,000 miles. The catalysts in the 1991 Escorts had Pd/Rh as the active metals.Our results clearly show the that the manganese oxides deposits on catalytic converters: 1) do not adversely affect performance; 2) do not adversely affect the surface area of the catalysts; and 3) inhibit the deposition of catalyst poisons such as phosphorus and zinc on the catalyst surface.