In chemistry isomerization (also isomerisation) is the process by which one molecule is transformed into another molecule which has exactly the same atoms, but the atoms have a different arrangement e.g. A-B-C → B-A-C (these related molecules are known as isomers ). 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 somewhat 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.