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The article presents the activities of Polish priests from the Congregation Sacerdotum a Sacro Corde Jesu (SCJ) in the Democratic Republic of Congo (Zaire) in the years (1970-2005) as seen from the perspective of the missionaries' correspondence. The chronological scope covers the years Polish priests took over Congolese parishes (Upper Congo) until they were entrusted to local priests in 2005. The correspondence documents the mission of evangelization, which the priests worldwide known as the Congregation of Dehonian Priests (after the founder's name) combined with social work performed in climatically, economically and politically difficult conditions. It is currently kept in the religious archives, constituting an important source for the study of the history not only of the Congregation, but also of Congolese society among which the priests carried out the ministry of the Catholic Church. Unlike official documentation, they reveal the missionaries' personal experiences, their unique vision of reality, which can become a subject of interest for representatives of many research disciplines: historians, anthropologists, sociologists, educators, cultural and literary scholars.