Father Peter Jan Beckx, Father General of the Society, summoned him to Rome to act as official theologian at the First Vatican Council. Father de Buck assumed these new duties with his accustomed ardor and, upon his return, showed the first symptoms of arteriosclerosis, which finally ended his life. Toward the end of his life, Father de Buck lost his sight, but dictated from memory, material previously compiled regarding saints of the early Celtic Church in Ireland, to his brother, Fr. Remi de Buck, also a Bollandist.