This paper studies the syntactic structure for adherence Adh (IM) of a language L. With the aid of the parser tree of L, the authors analyse some characteristics of words belonging to Adh (L), and prove iteration theorems for adherences of regular languages and context-free languages, thus solving the problem on syntactic structure for the adherence of corresponding languages. Moreover, they present the application of itera-tion theorems to prove some adherence of languages are not adherences of regular languages or context -free languages.