Hydrogenation is a class of chemical reactions which result in an addition of hydrogen (H 2 ) usually to unsaturated organic compounds. The process constitutes the addition of hydrogen atoms to the double bonds of a molecule through the use of a catalyst. Hydrogen also adds to triple bonds if they are present. Typical substrates include alkenes, alkynes, ketones, nitriles, and imines. Most hydrogenations involve the direct addition of diatomic hydrogen (H 2 ) but some involve the alternative sources of hydrogen, not H 2 : these processes are called transfer hydrogenations. The reverse reaction, removal of hydrogen, is called dehydrogenation. A reaction involving hydrogen and cleavage of a carbon-oxygen bond or carbon-nitrogen bond is called Hydrogenolysis. Hydrogenation differs from protonation or hydride addition (e.g. use of sodium borohydride): in hydrogenation, the products have the same charge as the reactants.

The classical example of a hydrogenation is the addition of hydrogen on unsaturated bonds between carbon atoms, converting alkenes to alkanes. A simple example is the hydrogenation of maleic acid to succinic acid depicted on the right. Numerous important applications are found in the petrochemical, pharmaceutical and food industries.

Health concerns associated with the hydrogenation of unsaturated fats to produce saturated fats and trans fats is an important aspect of current consumer awareness.