Among the twenty-two anonymous masses contained in the so-called Alamire manuscripts is an unedited canonic, five-voice Missa Cueur langoreulx. Th e mass is unique to MontsM 766 (a manuscript thought to have been sent to Charles V in Spain). The model for this mass is identified as a chanson by Josquin Desprez and the relationship between the mass and its model is examined. An analysis of MontsM 766 suggests a purpose and theme of the manuscript, and situates the Missa Cueur langoreulxin the context of its sole source. In order to determine the composer’s working methods and to relate the mass to sixteenth-century polyphonic tradition, this paper explores the anonymous composer’s treatment of his borrowed material, as well as the modal, mensural, and thematic structure of the mass in comparison to that of the chanson. In addition to bringing a previously unknown mass based on a little-discussed chanson by a major Renaissance composer to attention, this study refines our knowledge of polyphonic borrowing and of techniques of canonic writing.