The second of the January volumes of the Bollandist critical hagiography known as the Acta Sanctorum was published in 1643 and it included the dossier on the fourteenth-century papal diplomat and Latin Patriarch of Constantinople, St. Peter Thomas O. Carm., compiled by Godfrey Henschen S.J. The dossier was in two parts, the first a critical review by Henschen of previous scholarship on the activity of St. Peter as papal legate and the second an edited transcript of the Life of St. Peter by his associate and friend, Philippe de Mézières, the Chancellor of the Kingdom of Cyprus, as found in the Cronicon Universale of Tierri Pauwels. Identifying the various later accounts of the Life of St. Peter Thomas and, in particular, those accounts by Carmelite authors that sought to represent him in a manner that was not consistent with the account by De Mézières, Henschen argues, for example, that the claim that St. Peter was martyred is inconsistent with the eye-witness acount of the last illness an death of the Saint by De Mézières. The Pauwels text on which Henschen bases his edition of the Life has, in more recent times, been recognised as flawed in some significant ways, and Henschen later revised the critical review that forms the first part of the present manuscript, but the 1643 dossier constitutes, nevertheless, the first truly critical study of the hagiography of St Peter and it continues to be the text that is most frequently mentioned and cited. This is the first English translation and edition of a document that is important, not only from the point of view of historiography, but also from the perspective of the history of the fourteenth-century. As far as possible, the various historical sources and individuals mentioned either in the Life of St. Peter or in the critical review by Henschen have been identified and indexed and notes have been provided to enable the modern reader to appreciate the significance of some obscure points in the text.