‘The Syon Pardon Treatise’ in London, BL, MS Harley 2321 is a Birgittine text from the second half of the fifteenth century. It has not previously been published and is unique except for eighteen lines found in Cambridge, Corpus Christi College, MS 156. The Harley text covers ninety-one pages and has the form of a sermon aimed at a lay audience. Birgittine sermons were delivered in the vernacular and restricted in length. However, the length of this text is such that it could not have been preached ‘ad populum’ in its entirety; the term treatise is more appropriate and has been used here. Harley 2321 appears to have been made for one or more brothers at Syon who were in contact with pilgrims there. The text provides a wealth of information on indulgences in late fifteenth-century England and insights that have relevance beyond Syon. The introduction contains detailed descriptions of the two manuscripts and discusses Birgittines and preaching, contemporary doctrine on indulgences and penance, and indulgences at Syon and other Birgittine foundations.