The article is devoted to the relations between the Halych Kingdom (since 1199 the Halych-Volyn Kingdom) and Byzantium. It focuses on the history of the region of the Lower Transnistria and the lower course of the Danube and their significance for Halych-Bizantine relations. The author presents the interests of both parties in relation to the above-mentioned region, explains the reasons for the exacerbation of relations between Halych and Constantinople in the second half of the 12th century, provides an explanation as to why the Halych prince Yaroslav Osmomysl supported Bulgarians against Byzantium, describes the renewal of Halych-Byzantine alliance under the rule of Roman Mstislavich and the assistance which the Ruthinians provided to the Bulgarian Tzar Ivan Asen II. The final section of the article describes the way in which Halych and Byzantium lost control over the Lower Transnistria and the lower course of the Danube and how these territories were taken over by Mongols.