What does it take for a reform to succeed? The 'Reformatio Sigismundi' (1439) seems to pose this question regarding its own literary form: its anonymous author presents his ideas in German while enriching them with vernacular elements from sermons, prophecies and political publicity. Thus, in the midst of the Latin debate on a reform of church and empire at the Council of Basel, emerges the very first comprehensive reform text in German. So far, it remains unanswered why the anonymous author chooses the vernacular and how the change of language affects the argumentation and intention of the text. This study is the first to examine the 'Reformatio Sigismundi' from a literary perspective testing different philological methods in working with historical sources.