This article is an attempt to establish the time-frame and relative chronology of the š > s and ö, ü > e, i changes that occurred in south-western Karaim. The sample material used for the present article comes from Halych Karaim handwritten prayer books dating back approximately to the second half of the 18th and the first half of the 19th century, and are written in the Karaim semi-cursive variant of the Hebrew script. The final conclusion of the article is that both changes occurred in the final decades of the 18th century.