In 1019–20, the bishoprics of Sirmium, Ras and Prizren (roughly corresponding to modern-day Serbia) are mentioned as the westernmost eparchies in the area of the Archbishopric of Ohrid.

[1] To the west of these eparchies lay a borderland with ecclesiastical provinces of the Metropolitanate of Dyrrhachium and Catholic bishoprics of maritime cities.

[1] The exact borders of the Theme of Sirmium are unclear: according to some sources, theme included region of Syrmia (on the northern bank of the Sava river) as well as parts of modern Serbia and Bosnia and Herzegovina on the southern bank of the Sava river,[2] while, according to other sources, it extended along the southern bank of the Danube and along the river Sava.

In the last years of the 12th century, Byzantine power waned, and the emergence of the Second Bulgarian Empire created a new contender for the region's control.

One of these states, the Kingdom of Syrmia, was centered in the area in which Byzantine Theme of Sirmium existed.