The paper explores the location where the founder of Arian heresy, Alexandrian presbyter Arius, was exiled by the emperor Constantine I after the Nicene Council in 325. It has been generally accepted among researchers that the place of Arius’s exile was “Illyricum”, and the notion was too wide in the Late Roman Empire in order to give more precise location. However, many renowned authors narrowed this area down to one of the imperial residences in the Western Balkans – Sirmium, Serdica, Naissus being the most common choice. The author of this paper critically re-examines the written evidence of Arius’s exile, including the archeological sources, so as to conclude that the written argumentation solely depends on a rather unreliable source (Philost. HE 1.9c). However, the paper offers a solution to the problem by comparing it to other indirect sources, such as the Roman penal code in the case of religious exile, the praxis of the exile among known cases, the relationship between Arius and later attested Arian bishops in Pannonia.