戒規
In 1862, he was appointed secretary to the first commission for the preparation of the "Syllabus". Later named a canon of the Lateran Basilica, Jacobini served as a referendary on the Apostolic Signatura as well. In 1867, he was charged with the weighty task of gathering and publishing the answers of the world's bishops to the questionnaire for the preparation of the First Vatican Council. During the Council, Jacobini was secretary of the preparatory commission for Church discipline, also serving as Undersecretary of the Council from 1869 to 1870.