‘Split or not to split infinitives’ still remains controversial among grammarians, native English speakers, and English learners, even though the grammatical construction was officially approved as correct. In this regard, this study aims to explore how split infinitives are used in the reading texts of high school English textbooks. To this end, a corpus was compiled, comprising all the reading materials from 18 high school English textbooks. From this corpus, all the examples of ‘split infinitives’ were identified using the concordance function of AntConc to see how many split infinitives occur in the English textbooks and what types of adverbials are used in the construction. The results indicated that the ratio of split infinitives to to-infinitives in the textbooks is similar to that in COCA, even though the split infinitives are not evenly distributed throughout textbooks analyzed in this study. In addition, the types of splitter adverbials that occur in the textbooks are likely to be adverbs of degree(e.g. really, simply, actually, better) and manner(e.g. carefully, creatively, easily), which are considered to mainly occur between to and verbs in the split infinitive phrases. Therefore, this study suggests that the high school English textbooks provide authentic reading materials that include language change regarding split infinitive use.