The Chamber Orchestra of Philadelphia and Orchestra 2001, groups with different mandates, played concerts on Sunday in which they could have been switching roles.

At the Kimmel Center, the relatively mainstream Chamber Orchestra had a Schoenberg/Mahler program only somewhat dissimilar to one that Orchestra 2001 played a few seasons back. Then at Swarthmore's Lang Concert Hall, Orchestra 2001, a new-music group, played a scaled-down program of Milhaud and Honegger, squarely in the Chamber Orchestra's zone.

The similarities came out of diminished ambitions, which is unfortunate. But, fortunately, what was on the programs were collections of mostly deserving pieces that should be heard more.

The Chamber Orchestra's program under Belgian guest conductor Dirk Brossé was dominated by Mahler's hour-long song cycle Das Lied von der Erde in an orchestral reduction begun by Schoenberg and finished by Rainer Riehn. Though Mahler's own orchestration is preferable, this version for string quintet, woodwind quintet, percussion and keyboards has lots less amplitude, so that singers need not be high-priced Wagnerians.

Tenor Jason Collins has sung Mahler's original, but no leather-throated vocalist is he. Rarely are those near-impossible vocal lines navigated without strain by such a fresh, lyric voice - and, in songs involving alcohol, a sense of fun. Mezzo-soprano Mary McCormick had more to live up to; her predecessors are the great art-song interpreters of our time. Her relationship with the final song, "Der Abschied" (about the ecstatic embrace of death), appeared to be deep, but didn't sound that way with pitch problems clouding what she was projecting.

Conductor Brossé drew an unusually solid performance, with minor miracles of phrasing and beautifully judged vibrato, from cellist James J. Cooper and, especially, concertmaster Gloria Justen. He also delivered a clear view of Schoenberg's 1934 Suite for String Orchestra, but to what purpose? The composer wears the mask of baroque-period manners and, disjointedly, lets it drop abruptly here and there. Schoenberg's other Suite (Op. 29) is better but more challenging atonal music, and would have been a better pairing with Mahler.

Orchestra 2001's program, Mysteries for Orchestra, initially promised Henri Dutilleux's Mystère de l'instant, a lovely series of moody micromovements. It was sorely missed, but left a more streamlined program, featuring Milhaud's La Creation du Monde, Honegger's Symphony No. 2 and, more important, a world premiere, Jay Reise's substantial The River Within (Concerto for Violin and Orchestra).

Reise, an esteemed University of Pennsylvania faculty member, composes with a demure but unmistakable and always inviting voice. In this unusually traditional, lyrical piece, he utilizes standard forms with fluidity and mastery that creates an optimum showcase for his attractive thematic ideas. Violinist Maria Bachmann was full of conviction, but maybe the wrong sort; with so much electricity and so little repose the piece felt dense and breathless.

The Honegger symphony, which often sounds the way overcast French-Swiss weather looks, took on unusual luster and passion in the final movement. While Milhaud's jazz-influenced ballet often receives dreamy, stream-of-consciousness readings, this one rocked, as if being played by Woody Herman's Thundering Herd. Music director James Freeman, on a good day, offers admirable objectivity and clarity. This time there was heat, and lots of it.