English Abstract This article examines the development of the library of the International Mozarteum Foundation after Austria's annexation by Nazi Germany, 1938 to 1945. Although the Mozarteum Foundation was under direct government control, it nominally remained a non-governmental institution even during the Nazi era. Therefore, although it was not directly involved in the National Socialist system of cultural property theft, it made intensive efforts to expand its holdings from collections expropriated by the National Socialists. The driving force behind this was the musicologist Erich Valentin, who was also responsible for the library; he worked closely with the Viennese professor of musicology, Erich Schenk. Various, entirely unsuccessful attempts to profit from the expropriation of Jewish collectors (like Stefan Zweig) are described. In contrast, the intensive efforts of the Mozarteum Foundation to take over expropriated libraries of ecclesiastical institutions, such as St Peter's Abbey in Salzburg with its valuable historical holdings, were successful. The article also deals with the restitutions after World War II, which only began a decade after the end of the war and were not carried out systematically. Therefore, looted property could still be identified in recent years, the restitution of which is also addressed. An important concern of the article is also to point out the importance of a thorough documentation of the results of provenance research (not only in regard to the Nazi period). All the information ascertained about the provenance of the individual sources should be permanently documented in the catalogues, if possible, in standardised form.
