This master’s thesis studies the reflex of yat in root morphemes in Meštrija dobra umrtija and Ritual. The aforementioned work was printed in the Glagolitic printing house that operated at the end of the 15th and in the first decade of the 16th century in Senj, founded by the local clergy. Basic liturgical books as well as Croatian translations of church manuals and devotional works popular in Europe at that time were printed there. Seven books were published in the period between 1494 and 1508. Meštrija doba umrtija was printed together with the Ritual. Two incomplete copies of this edition have been preserved, both without the beginning and the colophon, so the exact date of publication is unknown, but typographical and technical similarities with other Senj editions indicate that the work was printed in the same printing house in Senj. Meštrija doba umrtija contains texts intended for priests, but also for laypeople who find themselves with the dying, while the Ritual is composed of the essential rites related to illness, death, and burial. In the previous literature, it is stated that the Meštrija was written in the Chakavian language of Senj (of the Ikavian-Ekavian type), and the Ritual in the Croatian redaction of Church Slavonic. In the paper, we will explore whether there are differences between them in the reflex of yat, aiming to contribute to a better knowledge of the literary language represented in the editions of the Senj Glagolitic printing house. In the first part of the thesis, the activity of the Senj Glagolitic printing house is presented with a special reference to the Meštrija dobra umrtija and Ritual, then the development of yat from the Proto-Slavic language to the Old Croatian language is shown and its reflexes in the Chakavian dialect are described. The central part of the paper presents the corpus, analyses the reflexes of yat, and finally, the conclusions are drawn.