Regioisomers are constitutional isomers that differ in the position of a group or a multiple bond and that can arise from the same reaction. They matter because the regiochemistry of a reaction is described in terms of which regioisomer is formed. For example, but-1-ene and but-2-ene are regioisomers differing in the position of the double bond, and an electrophilic addition may give one in preference to the other.

区域异构体是在某一基团或多重键位置上不同、且可由同一反应生成的构造异构体。它们之所以重要,是因为一个反应的区域化学正是用生成哪一种区域异构体来描述的。例如,1-丁烯和 2-丁烯是双键位置不同的区域异构体,而某个亲电加成可能优先生成其中之一。