流变分子（fluxional molecule）是指内部原子在对称的位置上不停交换的一种分子。

因为事实上，从某些方面看，所有的分子都在流变，例如有机化合物中键的旋转，所以流变这个概念主要取决于上下文的关系以及检测的手段。

一般来说，当一个分子的光谱信号因为化学交换作用存在超出海森堡不确定原理所预测的行展宽时，我们说这个分子是一个流变分子。在某些例子中，由于流变的速率很低不能通过光谱检测，此时就会用同位素标记法。

常见的流变分子：

Fluxional molecules are molecules that undergo dynamics such that some or all of their atoms interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in some respects, e.g. bond rotations in most organic compounds, the term fluxional depends on the context and the method used to assess the dynamics. Often, a molecule is considered fluxional if its spectroscopic signature exhibits line-broadening (beyond that dictated by the Heisenberg Uncertainty Principle) due to chemical exchange. In some cases, where the rates are slow, fluxionality is not detected spectroscopically, but by isotopic labeling.