Most of the chants on which the pieces in Choralis Constantinus II are based are notated on their standard modal finals in the chant sources. Many appear in transpositions to the upper fourth or fifth in Isaac's settings. Isaac follows no standard procedure for modal transpositions. Every mode appears on at least two different finals, and mode 2 appears on three, in his settings. The same chant may even appear on different finals in different settings.
The distribution of modes, finals, and signatures in the collection is as follows:
Introit 12 (Nunc scio vere) and Introits 16, 22, and 24 (Sacerdotes tui) are based on chants classified as mode 3 in modern liturgical books, but they use the mode 4 verse formula in the Graduale pataviense and in Isaac's settings. They are classified as mode 4 on A above.
Alleluia 16 (O Geberharde) is notated on C with some B-flats and some B-naturals in Sankt Gallen, Stiftsbibliothek, Cod. Sang. 546. It could have been classified as either mode 6 or mode 8 in that source. It is classified as mode 8 on G above.
Alleluia 23 (Felix virgo) is notated on F with B-flats in Sankt Gallen, Stiftsbibliothek, Cod. Sang. 546. It would have been classified as mode 6 in that source. Isaac probably had a source in which it was notated on G, in which case it would have been classified as mode 8. It is classified as mode 8 on G above.