The article deals with the toponyms occurring in the Aramaic and Arabic texts of the Late Sassanian and Early Moslem period concerning the biography of the prominent Eastern Syrian mystical writer Isaac of Nineveh. Two particular cases are analysed. Firstly, it is reported by Ishodnah and other writers that Isaac left Qatar in the mid-7th century and became bishop «of Nineveh», whence his cognomen Ninwāyā. The history of Nineveh and its mythological reception are traced to the 7th c. BC. Due to the never forgotten glory of Assyrian past, any new centre which ever re-emerged at Kuyunjik or Nabi-Yunus hills (which had been parts of Assyrian Nineveh) and even the pre-Mosul settlement on the opposite bank of the Tigris (once called Nav-Ardashir) received the name of «Nineveh» and were thought to be the same Assyrian Nineveh. It was this western pre-Mosul settlement that is really implied by «Nineveh» of Isaac. The population lived on the western bank of the Tigris in Nav-Ardashir, while the historical city of Nineveh had been abandoned. Bishops of Nineveh resided in the monastery of Beth Abe (in the Forests). It can be concluded that the term Ninwāyā in the episcopal title of the Church of the East was a mere convention. Secondly, the toponym Matut is brought under analysis. After leaving Beth Nuhadra Isaac moved northwards to Susiana (Beth Huzaye), where he spent some time in the monastery of Rabban Shapuhr before moving to the mountain cave where he spent the rest of his hermitic life. The name of the mountain in Aramaic sounds Matut and it is said that Matut encircled Susiane which makes «Matut Mt.» to be a rather vast segment of Zagros. It is impossible to explain the horonym quite reliably, but it can be hypothetically interpreted as a late form of Ancient Mesopotamian GN Mat-Utem (a part of Zagros region at upper Lesser Zab was called that as early as the 2nd mill. BC), used in extended sense.