“New Light on the Anonymous Mass Cycles in Prague, Czech National Library, Ms. 59 R 5117” In 1994, the Czech National Library acquired a previously unknown sixteenth-century choirbook of central European provenance. Now housed under the shelfmark 59 R 5117, the source contains eight four-voice mass-cycles, as well as choral settings of the mass-responses. Of the masses, three have up to now been considered unique: a Missa Presulem ephebeatum, attributed to Heinrich Isaac; and two anonymous, title-less cycles. This paper evaluates the latter two cycles for what they reveal concerning the source that contains them and the circulation of music in central Europe at the time in which the source was produced. One of the anonymous masses is chant-based. Martin Horyna, the first, and, to date, only scholar to study Ms. 59 R 5117 in any detail, identified this cycle as a Missa dominicalis in a short study from 2002. Since then, the mass has received no further scholarly examination. My paper identifies concordances and a composer for the mass. The other anonymous cycle can be identified as having been modelled on a motet with the text Vulnerasti cor meum. It too turns out not to be unique. The mass bears the hallmarks of having been composed in western Europe, possibly at the French royal court. I assess the piece, its sources, compositional techniques, and model. David J. Burn studied music at Merton College, University of Oxford. He completed his doctorate in 2002 on Heinrich Isaac’s mass propers under the supervision of Reinhard Strohm. From 2002-2003 he was Guest Researcher at Kyoto City University of Arts. From 2003-2007 he was Junior Research Fellow at St. John’s College, University of Oxford. In 2007 he joined the University of Leuven musicology department as head of the Early Music Research Group. His research focusses on the later 15th and 16th centuries, with particular interest for Heinrich Isaac and his contemporaries, interactions between chant and polyphony, source-studies, and early-music analysis. He has published widely on these topics in leading international peer-reviewed journals including the Journal of Musicology, the Revue de musicologie, the Archiv für Musikwissenschaft, the Journal of Music Theory, and Musiktheorie. In 2020 he was Queen Wilhelmina Visiting Professor at Columbia University in New York City, and in 2023 he held the Pieter Paul Rubens Visiting Professorship at UC Berkeley.