Regiochemistry is the aspect of a reaction concerned with which of two or more possible positions a group adds to or forms at, since different choices give different constitutional isomers. It matters because many additions and eliminations could in principle give more than one product, and the regiochemistry decides which is obtained. For example, in the addition of H–X to an unsymmetrical alkene, the regiochemistry determines on which carbon the hydrogen ends up.

区域化学是反应中涉及基团加到(或形成于)两个或多个可能位置中的哪一个的方面,因为不同的选择会给出不同的构造异构体。它之所以重要,是因为许多加成和消除原则上都可能生成不止一种产物,而区域化学决定得到哪一种。例如,在 H–X 对不对称烯烃的加成中,区域化学决定氢最终落在哪个碳上。