As a scholar Toustain became highly respected. He worked for twenty years with his fellow-Maurist, Dom René-Prosper Tassin, on an edition of the works of the Byzantine Greek monk Theodore the Studite. It was never printed, as a publisher could not be found. Another joint project of the two was the Nouveau traité de diplomatique (6 vols, 1750–1765) in which they addressed more fully and thoroughly the subjects taken up in Jean Mabillon's great work De re diplomatica. Toustain and Tassin's work provided the basis for the modern discipline of diplomatics. The final four volumes were edited by Tassin alone following Toustain's death.
