The project concerns with the critical edition of the mise en prose of “La chanson de la croisade contre les albigeois” from the fourteenth century. Written by an unknown author, it was transmitted in three manuscripts: Paris (BNF fr.4975, sec. XVI), Tolouse (Bibliothèque Municipale, ms.608, sec. XVI) and Carpentras (Bibliothèque Inguibertine, ms.1826, sec. XVII). A comparison between this text and poetical source, published by Eugène Martin-Chabot and based on the unique ms. BnF 25425, is also planned. This work seeks to establish the correct text of the mise en prose and to critically examine the language, a late Occitan far from the standard form used by the troubadours.