Because Senn's life was primarily devoted to scholarship and teaching, the bulk of the collection consists of scholarly and personal correspondence detailing the private activities of colleagues and friends, academic appointments, teaching duties, publication projects, study and travel abroad, and the affairs of professional organizations.
Roughly half of the collection consists of foreign language materials. Most of this is in German (either typed or neatly written), but there are also numerous Lithuanian pieces both from the United States and abroad as well as a few others in such languages as French, Italian, Polish, and Russian.
Apart from the Manuscripts series, almost all items come from Senn's correspondents. Possibly the most important items among the correspondence include 1938-1939 materials relating to Swiss-American cultural affairs and problems of the Swiss-American press in resisting pan-German encroachments (box 3, folders 1-9; box 4, folder 1), letters of the late 1930s detailing the problems of German scholars such as Alfons Nehring victimized by the Nazi regime, and Senn's efforts to find them suitable employment in the United States (box 2, folder 2; box 3, folders 1-2; box 8, folder 5), and correspondence from friends and Lithuanian displaced persons in Germany describing postwar conditions in that country (box 4, folders 6-9; box 5, folders 1-4).
