During the eleventh century the Aquitanian monastery of St. Yrieix, located forty kilometers south of Limoges, acquired a new gradual, a manuscript containing the liturgical Mass chants for the year. The Gradual of St. Yrieix, now at the Paris Bibliothèque Nationale (Pa903), includes both text and music, redacted with the musical notation typical of the region of Aquitaine. The objective of this research is to analyze Pa903 as a document of liturgical musical practice and as a participant in the historical events of its region and time. While the Gregorian chant repertory dominates the gradual, this dissertation addresses the neo-Gregorian chants of Pa903, composed in the period following the dissemination of Gregorian chant throughout Europe. These neo-Gregorian chants were open to the influence of the contemporary regional musical style and cultural traditions surrounding St. Yrieix. Chapter II reviews the backdrop of historical events surrounding Pa903, focusing on the reform and expansion at St. Yrieix and its transition from a monastery to a chapter of canons. The musical and liturgical characteristics of Pa903 (Chapter III) show that St. vii Yrieix favored its senior patron St. Martin of Tours and St. Aredius (its patron saint) above St. Martial of Limoges (a powerful neighbor) and presented in the gradual a community of saints with strong regional influence. Chapters IV and V analyze the concordances of antiphons, tropes, prosulas, prosas, and neo-Gregorian Mass chants of Pa903 with those of the Aquitanian graduals and other sources throughout Europe. The tropes of the Proper and Ordinary, the complete repertory of prosulas and prosas, and the neo-Gregorian Mass chants of Pa903 are collected together here for the first time outside of Pa903. The neo-Gregorian chants are found in the sanctoral, temporal, and the ritual Masses and include a group of chants that reflects textual and musical elements of the prior Gallican tradition. The chant repertory of the gradual also presents a subgroup of forty-nine antiphons, prosas, prosulas, and neo-Gregorian Mass chants found only in Pa903, documented here with musical examples. / text