This paper examines the phonetic value of the semivowel in Gavrilo Stefanović Venclović’s Služabna knjiga. The work was written between 1711 and 1716 in the Old Church Slavonic language using old Cyrillic script. The script is of the semi-uncial type with elements of cursive writing. The analysis is based on photographs of the manuscript (РГБ, Собрание Н. П. Румянцева Ф. 256 № 401). The text was transcribed using the Transkribus software platform, while most of the excerpted examples were processed using the AntConc software. The main findings of the research can be summarized as follows: (a) semivowel signs and the apostrophe at the end of a word have a purely orthographic function (e.g. даръ, хотѣщимь, готов); (b) as in the vernacular, in the Serbian Slavonic language epoch, the semivowel in a strong position generates the reflex a (e.g. вѣнацъ, диванъ, четвртакъ); (c) the secondary semivowel is vocalized as a (e.g. оганъ, петарь, жизан), or not recorder at all (e.g. жизнъ, огнъ, помыслъ); (d) the semivowel in a weak position, according to the rules of the Serbian Slavonic linguistic norm, is pronounced as a within the prepositions въ (e.g. въ бꙑтїе), съ (e.g. съ нами), and къ (e.g. къ г҃ꙋ), prefixes въ- (e.g. въходиⷮ), въз- (e.g. въⷥдиханїе), and съ- (e.g. съдѣлавъ), and the root въс- (e.g. въсакаа), as evidenced by examples where the letter а appears in place of the former weak position semivowel (e.g. васака, множаствѣ, саблюденїе)