The old book collection of the University Library of Cluj-Napoca contains items of mixed origins: The library of the Transylvanian Museum Association, the book collection of the university, the library of the Reformed College o f Orăștie (Szászváros) etc. The goal of this paper is to present a segment of this diverse collection: We have selected fifteen protestant theologians, so-called “protestant reformers,” who were active during the 16th century: Martin Luther, Philipp Melanchthon, Georg Major, Johann Brenz, David Chyträus, John Calvin, Thédore de Béze, Pietro Martire Vermigli, Heinrich Bullinger, Rudolf Gwalther, Ludwig Lavater, Johannes Oekolampad, Wolfgang Musculus, Benedictus Aretius and Girolamo Zanchi. I have examined a total of 97 prints from this theologians. Nearly half of these volumes were in the Carpathian Basin before 1600. Among the owners of that time, there is a similar number of Transylvanian Saxons and Hungarians from the Partium and Transylvania, the former in slightly larger proportions. Throughout the 17th century, these were the most important groups of owners as well, but now the group of the Transylvanian Reformed Hungarians is the largest.