This is a commentary in the form of homilies on each of the twenty-two stanzas of Psalm 118.

Fragments of the work (VI.xxi ff. and VII.vii ff., see Petschenig, CSEL 62.2) occur in four ninth-century fly leaves from a manuscript written at Nonantola (see Keynes 1992 p 28; Bischoff 1983 pp 114-16) added to the Trinity College manuscript of ARATOR's HISTORIA APOSTOLICA According to Lapidge (ML p 61) the Arras manuscript (saec. ix) is the book referred to in the Saewold booklist as "Librum expositionis Ambrosii de psalmo CXVIII."

Kitson 1983 (pp 95-99) argues that parallels cited by Meier 1977 (1.180 (note 207), 207-8, 287-88, 353 and 359) between Ambrose's commentary and BEDE's discussion of the topaz in his COMMENTARIUS IN APOCALYPSIM show that Bede did not use Ambrose directly, but depended on "an extended gloss drawn from Ambrose in the 670s or 680s by one of the less gifted students at Theodore's school" (pp 97-98).