异构化反应也称异构化，指某种化学物质在特定条件下改变自身的组成结构，从而成为新物质的反应。产物通常是反应物的异构体。许多异构体的键能相差不大，因此在常温下可相互转化。

• 白磷在高温下变为红磷，红磷在高温下又可重新变为白磷。
• 戊烷在金属铂的催化下，可转化为2-甲基丁烷和2,2-二甲基丙烷。
• 逆转醇的反式异构体暴露在紫外线下，可转化为顺式异构体。

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.