This is a study of five offertory antiphons and the prosulas written to fit them (and in some cases to fit their verses as well). The original focus of the investigation was three of the prosulas written on the last folio of the 10th-century manuscript Apt, Basilique Sainte-Anne, 18 (shown on Plates 1 and 2). The study was then enlarged to bring in prosulas for two offertories whose melodies are similar to those first investigated. There is a special point in studying these particular compositions, since the four prosulas central to the discussion are rare examples of prosulas for the antiphon of the offertory, rather than its verse. But they are treated as broadly as possible in this paper, and made the subject of comment on the origin and nature of the base antiphon, its text and music, the function of the prosulas, the technique of their composition and the relationship between their text and their music. A critical edition and translation of each prosula is provided, and the first of these is preceded by a brief discussion of the problems of editing prosula texts. It is hoped that a fully rounded view will thereby be provided of the offertory antiphon and the prosula as a musical, literary and liturgical unit, and that fresh insights into their composition will be gained.