This study seeks to trace out the structure of the Book of Prophet Isaiah with commentaries and to explore what that structure reveals about the text in some manuscripts of the East Slavonic and South Slavonic traditions. There are three conclusions made as a result of the present study. Firstly, the analysis of the structure and the identification of the readings in Catena Slavonica in Isaiam shows a translation of a catena which occupies an intermediate position between the Catena in Isaiam by John Drungarios and the one by Andrew the Presbyter whichever is the earliest. The CSI resembles both. Secondly, the value of the CSI should not be underestimated, because it includes a translation of scholia by Theodulus whose work is now almost entirely lost. Therefore the CSI could provide new evidence for the content of the lost Byzantine original of Theodulus’ Commentary on Isaiah. Thirdly, the comparison of the numerals in the margin of РНБ F.I.461 with the sequence and number of the biblical pericopes and relevant scholia in the Russian manuscripts clearly and unequivocally demonstrates that although F.I. 461 is the earliest evidence of Preslav translation in a Tărnovo redaction, it is still a single link in the chain of the Slavonic tradition and has a many shortcomings compared to the CSI in the Russian tradition.