The Kevserî Mecmûası is an eighteenth-century Turkish manuscript, which includes, in addition to a variety of music-related contents, a fairly large collection of notations. Given the scarcity of notational sources, it offers invaluable material for understanding the performance practice and compositional style of Ottoman music before the nineteenth century. Having studied a hitherto unknown microfilm copy of this manuscript, the original of which has remained up to now in a private library and closed to our access, the author aims in this article to introduce this source and discuss its potential contribution to the knowledge of Ottoman music. The article is organised roughly in two parts. First, it will present briefly the contents of the manuscript, integrating new evidence and clues about its date, author and purpose into existing knowledge. Second, it will focus on the collection of notations in the manuscript, and by comparing it with the earlier collections will argue that, contrary to the prevailing views expressed in the literature, no large-scale transformation in the style of Ottoman music took place in the early eighteenth century.