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Emissions level scatter between engines in quantity production and demands for continuing reductions in exhaust emissions, may well contribute to the widespread use of the through flow oxidation catalyst to enable the heavy duty diesel vehicle engine to meet the US particulate emissions limits after 1994. This paper deals with the development of the catalyst formulation to achieve substantial reductions in the volatile content of the particulate, while achieving selectively low conversion of fuel sulphur to sulphate. The question of whether the catalyst alone will bring a US 1991 certified engine into conformity with the more stringent 1994 limits is addressed, together with the effects of short term ageing. Finally, the relationship of the fuel aromatic content and sulphur level to the relative conversion efficiencies of HC and CO are considered.