At the beginning of the 20th century it reportedly had an area of 11,522 square miles (29,840 km2), while the preliminary results of the first Ottoman census of 1885 (published in 1908) gave the population as 388,625. The accuracy of the population figures ranges from "approximate" to "merely conjectural" depending on the region from which they were gathered.
Bitlis and Mus were formerly included in the eyalet of Erzurum. In 1875 they were detached and made a separate vilayet. The sanjak of Siirt was joined to the vilayet of Bitlis in 1883-84.