Hydrogenation typically refers to processes through which liquid vegetable oils are converted to solid or semi-solid fats, such as margarine. It refers to a chemical reaction in which "unsaturated" bonds between carbon atoms are "reduced" by attachment of a hydrogen atom to each carbon. The process thus results in the "saturation" of the atoms and, when carried to completion, converts unsaturated fatty acids to saturated ones. Processes accomplishing the reverse are called "dehydrogenation" or "partial dehydrogenation."

A side effect of incomplete hydrogenation, which has implications for human health, is the isomerization of unsaturated carbon bonds. The cis configuration of these double bonds predominates in the unprocessed fats of most foods. But hydrogenation often converts these molecules to trans isomers, which in fats have been implicated in heart disease ( see trans fats ).

Hydrogenation typically uses hydrogen gas as a reactant and an undissolved (or "heterogeneous") metal catalyst, such as nickel, palladium or platinum. Otherwise, the "homogeneous" rhodium-based catalyst known as Wilkinson's catalyst is often used. Such reactions belong to organic chemistry.