Research conducted for the publication of the cartulary of the old University of Leuven has brought up new information on the genesis of the charter of foundation for the old University of Leuven (9 December 1425). It appears that the text was almost copied from the text of the charter of foundation for the University of Rostock (13 February 1419). The comparison of both texts shows that changes made reflect the different condition in founding both universities. This article clearly shows that papal charters of foundation for universities are open to interpretation as sources for university history. Only when they are studied compared to other charters that were granted to universities, embedded in what we know about the formulary and the organisation of the papal chancery, it is made possible to make a distinction between chancery formula and scarce but valuable indications of specific input by interested parties at the time of the founding of the university. *The theme of the paper has been influenced by recent research conducted at the Katholieke Universiteit Leuven by Erik van Mingroot and myself. This research is part of a long term project, to be carried out by E. Van Mingroot, J. Roegiers & M. Nelissen, and aiming at the publication of the cartulary of the old University of Leuven, from its foundation in 1425 up to its abolition in 1797.