The Kitab of Sibawayhi, edited in the 8th century, is the oldest Arabic grammar in existence. It has gone through many recensions, but all are based on a copy from the 18th century. The author of this text has studied 77 manuscripts of the Kitab , of which more than 40 were used to contribute to a new understanding of the history of the text. She has now shown that all of the recensions, oriental and occidental, follow closely the - now lost - copy of al-Mubarrad (c. 14th century), which functioned as a true vulgate. Only one manuscript, found in Milan, managed to escape its influence and adhere to the rival Kufan grammatical tradition. This manuscript provides a new version of the text and makes a fresh reading of the Kitab possible.