A Diesel Oxidation Catalyst (DOC) is a diesel oxidation Catalyst (DOC) is a device added to the exhaust system of a Tractor that reduces the amount of particulate Matter (PM) emitted.

The device is generally a large honeycomb structure with an active catalyst made up of a layer of precious metal. Emission reductions are approximately 20\% for PM, 40\% for hydrocarbons and 50\% for carbon monoxide. DOC"s have a negligible impact on tractor fuel efficiency. Costs are dependent upon the size of the engine and are usually between 1000 € and 2000 € per unit, making them an ideal retrofit device.