A number of ancient Syriac grammars are analysed, as far as the passages related to diathesis are concerned. It appears that the concept and the definition of diathesis vary diachronically, but also according to the theoretical framework chosen by the authors. The influence of different Greek and Arabic models (and of their interactions) causes variation in the perspective under which diathesis is conceived and described. Particular attention is devoted to the middle diathesis, which is attributed to Syriac exclusively in the translation of the Téchne Grammatiké, made by Huzaya in the VI century. The comparison of the Greek original with the Syriac translation may also shed some light on the concept of middle diathesis, as meant in the Téchne.