1. A process involving the heterogeneous reaction of adding hydrogen to molecules containing carbon such as unsaturated animal and vegetable oils. Highly unsaturated vegetable oils can be converted to solid vegetable fats by catalytically hydrogenating some or all of the double bonds in a process known as hardening. Margarine is produced from the hydrogenation of unsaturated vegetable oils such as soybean, peanut, or corn to produce the consistency of butter. The product can be churned with milk and artificially coloured to give the appearance or flavour of butter.

2. Synthetic oil that is made by reacting coal with hydrogen to form synthetic hydrocarbons. See fischer–tropsch process.