The rivalry between Russia and Britain over the autonomous Province of Eastern Rumelia is a historical fact. This study examines tis fact via two historical events concerning the development of the Province – the process of establishment and administrative organization of Eastern Rumelia (1878-1879) and the event of unification between the Province and the Principality of Bulgaria. It argues that the British authorities created the Province of Eastern Rumelia and later they defended the act of unification with the purpose to thwart the Russian political advancement in the Balkans. The British political aim was successfully accomplished as Russia achieved to preserve its political domination in the Province only until 1885 when it opposed the act of unification. This research is based on archival documentary sources and secondary sources. The archive collections used for preparation of this work are the collections of the Foreign Office of the National Archive. The existing historiography on the issue of Eastern Rumelia is very limited as regards the variety of examined topics, though a considerable number of researches are dedicated to the act of unification and the political, cultural and economic development of the Province from 1878 to 1885.