A process used to remove sulphur from crude oil and refined petroleum products, and also the removal of hydrogen sulphide from natural gas. It involves a catalytic reaction in the presence of hydrogen. The use of bacteria to remove sulphur from crude oil has also been developed. It is important to remove sulphur as it poisons catalysts and results in harmful sulphur dioxide in the combustion of fuels. Also known as hydrodesulphurization, the process involves a high temperature and pressure reaction in the presence of a nickel or alumina catalyst impregnated with molybdenum.