[6] Only after the death of Theodosius in 395 and the division of the Empire did the Illyricum assume the permanent form which appears in the Notitia, incorporating the dioceses of Macedonia and Dacia, with Thessalonica as capital.

However, the Western Empire, especially during the regency of Stilicho, continued claim them until 437 when, as part of the dowry of Licinia Eudoxia, Valentinian III recognized the East's sovereignty over the prefecture.

[10] Following the Slavic invasions in the 6th and 7th century, most of the Balkan hinterland was lost by the Byzantines, who only retained control of the parts of Thrace nearest Constantinople, Thessalonica and its environs, and some coastal strips in Greece.

A praetorian prefect (ὕπαρχος) is attested in the sources as governor of Thessalonica as late as the first years of the 9th century, one of the last survivals of the old Constantinian administrative system in the entire Empire.

At that point however, the wars with the rising power of Bulgaria necessitated a reorganization of the provinces, and Thessalonica was constituted as a distinct theme under a strategos sometime before 840.