Hydrogenation is the process of addition of hydrogen. Unsaturated hydrocarbons are added with hydrogen in the presence of palladium and nickel catalysts to give saturated hydrocarbons.

${}_{_{R}/}^{^{R}\backslash }C=C_{{{\backslash }_{R}}}^{{{/}^{R}}}$$\xrightarrow[Ni\,\,Catalysis]{{{H}_{2}}}$ $\begin{matrix}
   \begin{matrix}
   H\,\,\,\,\,H  \\
   |\,\,\,\,\,\,\,\,\,|  \\
\end{matrix}  \\
   R-C-C-R  \\
   |\,\,\,\,\,\,\,\,\,\,|  \\
   R\,\,\,\,\,\,\,R  \\
\end{matrix}$

This reaction is applied in the hydrogenation of vegetables oils, which contain long chains of unsaturated carbons.