The reaction in which the addition of hydrogen occurs.

Hydrogenation is a chemical reaction between molecular hydrogen and an element or compound, usually in the presence of a catalyst. The reaction can be one in which hydrogen simply adds a double or triple bond connecting two atoms in the structure of the molecule, or one in which the addition of hydrogen leads to the dissociation (destruction) of the molecule (called hydrogenolysis or destructive hydrogenation). Typical hydrogenation reactions include the reaction of hydrogen and nitrogen to form ammonia and the reaction of hydrogen and carbon monoxide to form methanol or hydrocarbons, depending on the choice of catalyst.