The paper constitutes the knowledge on the status of the Danube Delta before and during the Russo-Turkish War of 1877-1878 for the first time in Ukrainian historiography and also it treats on the diplomatic efforts to give the Danube Delta over the Romania by the Sublime Porte in 1876. The article touches upon the issue of the Romania’s struggle in 1876 for revision of the delimitation of Russo-Turkish borders in the area of the Lower Danube and the changeover of the part of Delta under the Romanian jurisdiction. And it also analyses the Russo-Romanian relations before and after the Russo-Turkish War of 1877-1878 and it deals with the issue of the changing of borders in Russia and Romania. It was shown that the proposals’ evolution of Russian diplomat Nelidov for the status of Delta in the Russian position for Bessarabian borders. The role of Ignatiev’s diplomacy for solution to the problem of the affiliation the islands of Delta and negotiation with Romanians for the changing the borders between Southern Bessarabia and Dobrudzha with Delta before the San-Stefano Treaty was evaluated. The work of Russo-Romanian commission headed by Polivanov for acceptance and handover the Southern Bessarabia and what connected with delineation of body of water in the area of Danube delimitation is especially emphasized.