At the time the commission arrived, Duruflé was working on an organ suite using themes from Gregorian chants. Duruflé incorporated his sketches for that work into the Requiem, which uses many themes from the Gregorian "Mass for the Dead." Nearly all the thematic material in the work comes from chant.
The work is set in nine movements. Interestingly, the Dies irae text, perhaps the most famous portion of the Requiem mass, is not set as a movement by itself as usual. Overall, however, Duruflé chose the calmer and more meditative texts from the requiem.
In the full score, the fifth movement, "Pie Jesu," has the only solo for the mezzo-soprano; in addition, even in the "organ-only" version of the Requiem, there is an obbligato cello solo. The baritone soloist has parts in the third movement, "Domine Jesu Christe," and the penultimate movement, "Libera me."
Duruflé left indications in the score that, for the baritone soloist at least, it was preferable to have the choir sing the solos instead. This has resulted in various forces being used in different performances, some with both soloists, some with only the mezzo-soprano, and some (such as Robert Shaw's Telarc recording) using no soloists at all.