互变异构是某些有机化合物的结构在两种官能团异构体间产生平衡互相转换的现象，相应的异构体则称为互变异构体。大多数互变异构都涉及氢原子或质子的转移，以及单键向双键的转变。互变异构体在平衡中的分布与具体的因素有关，包括温度、溶剂和pH值等。

互变异构可被以下因素催化：

常见的互变异构包括：

Tautomers are constitutional isomers of organic compounds that readily interconvert with each other. The chemical reaction interconverting the two is called tautomerization. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond. The concept of tautomerizations is called tautomerism. Because of the rapid interconversion, tautomers are generally considered to be the same chemical compound. Tautomerism should be distinguished from resonance, where two structures differing only in the position of electrons coexist in quantum superposition, rather than as a rapidly-interconverting mixture. Tautomerism is a special case of structural isomerism and can play an important role in non-canonical base pairing in DNA and especially RNA molecules.