There are about ten letters of Isidore of Pelusium translated and copied in Syriac canonical collections (as found in Vat. syr. 560 and Charfet, Rahmani 87), reading mainly as regulations for different holders of offices (what should an archdeacon, priest, bishop, archon, the emperor, etc., do) In this paper I would like to explore the rationale as to why these particular Isidorian letters were included in the canonical collections, and to what extent they were indeed the best candidates for that. I will also tackle the question of why a later copy would lose or remove the letter to the emperor (Theodosius II) in particular, and in conclusion compare these data with similar practices in Latin collections.