The Gotha court and the Electoral-Mainzian governor's office in Erfurt under Karl Theodor von Dalberg (1744–1817) played an important role in the reception and dissemination of Enlightenment ideas in the 18th century. Evidence of this fruitful reception can be found in the private libraries of the duchesses, dukes and the governor, which are now kept in the Gotha Research Library and the Erfurt University Library. As part of the project “Cataloging the Private Libraries of the Age of Enlightenment”, funded by the German Research Foundation, the approximately 25,000 titles from the private libraries were recorded in the online catalogue and in the manuscript database of the research library according to the standards of the Working Group on Early Printed Books of the Common Library Network. They were indexed by subject according to the classification system of the ducal collection and by provenance. Selected key pages of each print were added to the bibliographic information as images. The handwritten catalogues of the private libraries are accessible online as digital documents in the Digital Historical Library Erfurt/Gotha and linked to the surviving titles in the online catalogue.