This paper sets forth some features of one of Bīrūnī’s works entitled Kitāb Bātanğal al-Hindī (The Book of Bātanjal the Indian, also commonly referred to as the Kitāb Pātanjal). Among the scant literature on this translation, there is agreement about the fact that there are notable differences between the Kitāb Pātanjal and Sanskrit literature on yoga known to us. This raises the question about the extent to which this work is a literal translation, and of what text. Some of these differences can certainly be explained as Bīrūnī’s own modifications and choices of interpretation; however, these choices have not yet been satisfactorily determined by scholars. Therefore, the aim of this paper is to throw light on some choices that Bīrūnī made when he decided to translate a yoga text associated with Patañjali into Arabic. Reflexions on the transformations due to these choices provide information on the form of the Sanskrit original text on the one hand and of the Arabic text on the other. The reasons, circumstances and consequences of Bīrūnī’s choices will be dealt with, as well as his method of translation. Prospects and limitations in the determination of Bīrūnī’s transformation of the original text will also be discussed. Thus, this first philological analysis aims at laying the foundation for a future examination of Bīrūnī’s understanding of the yoga system of Patañjali.