The aim of this work is to analyze the church administration position of northern Slavonia and south Baranya from the end of World War I until the area's integration into the structure of the Diocese of Đakovo or Bosnia and Syrmia. This also implies an analysis of the role and meaning of the Apostolic administration of northern Slavonia and Baranya as a church administrative unit that has been founded in that area in 1923. The introduction therefore provides and overview of previous research of Apostolic administration of northern Slavonia and Baranya, followed by establishing the theoretical and methodological research sources. The introduction is continued by a brief referencing of earlier initiatives to address the issues of church administration over northern Slavonia, as well as an analysis of international circumstances that have caused the formation of the Apostolic administration of northern Slavonia and Baranya. The focus is put on the Treaty of Trianon which divided Baranya between the Kingdom of Hungary and the Kingdom of Serbs, Croats and Slovenes. It ends with a description of Baranya's position within the Kingdom of SCS / Kingdom of Yugoslavia as the newly-emergent state and entity. The whole work is based on the combination of the synchronic and the diachronic principle. The central part of the work consists of three chapters chronologically connected by crucial decisions related to the territorial organization of church administration in northern and Slavonia and southern Baranya. Each chapter consists of a series of thematic subsections, one of which includes a review of international political developments that affected the church-state relations, while few of the chapters provide a statistical overview of data from relevant sources, such as diocesan schematics and canonical visitations. Also, each chapter provides several subsections concerning the timeline of the church-state relations at the aforementioned apostolic administrations' micro level. Some subsections show some other specific factors in the domain of church-state relations, which have significantly influenced both church and state policies. The first chapter begins with the emergence of the Apostolic administration of northern Slavonia and Baranya in 1923 and ends with its division into the Apostolic administration of northern Slavonia under the authority of bishop of Đakovo and the Apostolic administration of southern Baranya under the authority of bishop of Pécs in 1941. The second chapter encompasses the shortest period, which often coincides with the existence of the Independent State of Croatia 1941 – 1945. 9 The third chapter refers to the longest period. It covers the period from the end of World War II to the annexation of the Apostolic administration of northern Slavonia and Baranya in 1972, including the crucial 1952, when there was a reunion of two apostolic administrations in the areas of the Diocese of Pécs and the Diocese of Đakovo into a joint Apostolic administration of northern Slavonia and Baranya, under the administration of bishop of Đakovo. The conclusion provides a synthesis of all the analyses and the general phenomena obtained through processing and analyzing sources and all relevant factors that affected the position of the Apostolic administration of northern Slavonia and Baranya in the context of church-state relations. There are also some paradigmatic constants and variables in the church-state relations, which are comparable and applicable to analyzing similar church administration questions on the territory of the Kingdom of SCS / Kingdom of Yugoslavia, Independent State of Croatia and the Democratic Federal Yugoslavia / Federal Republic of Yugoslavia / Socialist Federal Republic of Yugoslavia.