Isomerization is the process by which one molecule is transformed into another molecule that has exactly the same atoms, but the atoms are rearranged. In some molecules and under some conditions, isomerization occurs spontaneously. Many isomers are equal or roughly equal in bond energy, and so exist in roughly equal amounts, provided that they can interconvert relatively freely, that is the energy barrier between the two isomers is not too high. When the isomerization occurs intramolecularly, it is considered a rearrangement reaction.
An example of an organometallic isomerization is the production of decaphenylferrocene, [(η-CPh)Fe] from its linkage isomer.
;Synthesis of fumaric acid
Industrial synthesis of fumaric acid proceeds via the cis-trans isomerization of maleic acid:
Topoisomerases are enzymes that can cut and reform circular DNA and thus change its topology.