hydrogenation (hĪdrôjˈənāˌshən, hĪˌdrəjənāˈshən) [key], chemical reaction of a substance with molecular hydrogen, usually in the presence of a catalyst. A common hydrogenation is the hardening of animal fats or vegetable oils to make them solid at room temperature and improve their stability. Hydrogen is added (in the presence of a nickel catalyst) to carbon-carbon double bonds in the unsaturated fatty acid portion of the fat or oil molecule:;g318;none;1;g318;;;block;;;;no;1;139392n;51425n;;;;;h-gnation318;;;left;stack;;;;;CE5Another hydrogenation is the synthesis of methanol from carbon monoxide. Hydrogenation reactions are important in petroleum refining; production of gasoline by cracking involves destructive hydrogenation (hydrogenolysis), in which large molecules are broken down to smaller ones and reacted with hydrogen. Most hydrogenation reactions are reversible and proceed to favorable equilibria at high pressure and moderate temperature.