One of the greatest challenges faced by musicologists is the proper identification and attribution of works preserved in manuscript sources. The aim of this article is to discuss this problem taking as an example Masses preserved in the Pauline Monastery at Jasna Góra (one of the largest collections of eighteenth- and nineteenth-century music in Poland) that are ascribed to Francesco Perneckher. The biography of this composer is still partly unknown, but between 1759 and 1768, he was employed in this monastery as a violinist and Kapellmeister. The Thematic Catalogue of Music Manuscripts and Prints of the Vocal-Instrumental Ensemble in Jasna Góra by Paweł Podejko lists six Masses by Francesco Perneckher. However, after careful analysis of the preserved sources, examination of the handwriting and comparison with the RISM database (presented in detail in the article), we may conclude that there are only two Masses of certain attribution: Missa Nativitatis Domini in A (shelf mark II-199) and Missa Nativitatis in F (shelf mark II-198). In the other cases, either the work was written by another composer (e.g., František Xaver Brixi) or else the preserved remains of the manuscript preclude a reliable identification.