The series of free concerts at the Library of Congress this week offered contrasting examples of rather old and rather new music.

On Wednesday night Vox Luminis, a recently formed vocal ensemble from Belgium in their Washington debut, performed a program of mostly baroque music from Italy. Though one could hardly complain about the unusual selection of rarely heard pieces, the execution, while generally fine, showed signs of vocal fatigue.

An unseen soprano opened the evening starkly with the unaccompanied “Lamentation de la Vierge au pied de la Croix,” a melody transcribed by scholar Yvonne Rokseth from a manuscript in the Bibliothèque nationale de France in Paris. This was beautiful, slightly mysterious music, sung in a metered rhythm, but it did not mesh all that well with the rather different Monteverdi madrigals that followed.

“Vorrei baciarti” featured the group’s two countertenors in a delicate duet, sometimes overpowered by the accompanying organ and viola da gamba, while the lower male voices were often too strong for their treble counterparts, as in “Alcun no mi consigli.”

Not surprisingly, the best music of the program, Monteverdi’s extraordinary “Lamento della ninfa,” prompted the strongest performance, presented as a quasi-operatic scene. The weaker “Lamentatio Virginis in Depositione” by Alessandro Della Ciaia foundered a bit in its long verses, as the soprano soloist’s intonation sagged slightly, but the double-chorus of angels kept things exciting. All of this led up to Domenico Scarlatti’s “Stabat Mater” for 10 voices, an extended orgy of juicy suspensions and angst-filled harmonic twists. The group’s recording of some of this composer’s sacred music, on the Ricercar label, is worth a listen. A single encore, Monteverdi’s “Christe Adoramus Te,” at the end of a long evening, pushed the soprano sound to the breaking point.

Downey is a freelance writer.