The purpose of the article is to study the peculiarities of the process of introducing the “Union of the Eastern Rite” in the eastern voivodeships of Poland in 1923-1937, the purpose of which was the religious assimilation of Orthodoxy and strengthening the infl uence of the Vatican. Th e author has analyzed archival documents, materials of the periodical press, and publications of the time, which highlight the problems related to the introduction of neo-union in the eastern voivodeships of Poland. Th e research methodology. Th e methodological basis of the article consists of the principles of historicism, objectivity, systematicity, and interdisciplinary approach. Th e general scientifi c, special-historical, and interdisciplinary methods are applied. Each of them has come to be eff ective in a specifi c situation and made it possible to objectively reconstruct the process of introducing the Eastern Rite. Th e scientifi c novelty of the work lies in the fact that the author, on the basis of archival documents and periodical press materials of the studied period, has identifi ed the peculiarities of the process of introducing the new union. Th e study is based on new source material, which is being introduced into scientifi c use for the fi rst time. In contrast to previous works devoted to this topic, the article more clearly reveals the dynamics of changes in the religious policy of the Polish state and the Roman Catholic Church in relation to the Orthodox Church. Conclusions. In the interwar period, the Orthodox Church faced a number of problems. One of them was the onslaught of Catholicism and the West’s at- tempt to subordinate Orthodoxy. Such was the so-called non-union action. One of the main reasons for the spread of the neo-Union movement in the eastern provinces of Poland was the decline in the authority of Orthodox priests, some of whom neglected their duties. Even the bodies of the dead could lie for a long time without a memorial service. Such priests viewed religion only as a source of enrichment and demanded high payment for priestly services. Such an attitude towards their pastoral duties undermined the authority not only of priests but also of the Orthodox Church itself. As a result, the population began to look for new ways to satisfy their spiritual needs. However, the neo-union movement still needed to gain mass distribution.