The Philadelphia Orchestra was on much newish ground Thursday with familiar composer names but not familiar pieces, chief among them that woolly bear of a choral work, the

Glagolitic Mass

of Leos Janacek.

I looked forward to seeing the facial reactions of the listeners around me to a piece so singular in the choral literature that you might not know what hit you. The piece is primitivistic yet modern, devotional yet secular, and so impulsive that even seasoned Janacek lovers are hard-pressed to know what comes next. What seemed logical to Janacek would be a mistaken non sequitur to his mellifluous compatriot Antonin Dvorak.

But far fewer faces than usual were nearby for reaction-monitoring. Even Simon Rattle is known to conduct Janacek concerts in Philadelphia to empty seats. And New York Philharmonic music director Alan Gilbert, this week's guest conductor, had a similar fate - a pity, as Thursday's performance will likely stand among the more thrilling of the season, though many of the performers were likely wrestling with the Glagolitic Mass for the first time.

The toll was apparent among the blue-chip lineup of vocal soloists - Tatiana Monogarova, Kelley O'Connor, Anthony Dean Griffey, and John Relyea - who weren't well-positioned for vocal projection and, in any case, struggled with the quirky vocal lines. The Philadelphia Singers Chorale, though, has rarely sung better, starting with the soft but intense first entrance that seized my attention and never let up.

Gilbert's artistic cool was a good temperamental match, shaping and building the piece's wild musical paragraphs but never actually taming them. Similar bravos also go to organist Michael Stairs: Though he effectively climaxed the piece with his imposing treatment of the organ solo, he also played with a subtle deliberation that clarified the music's progression of thought.

Other works on the program - Jean Sibelius' Night Ride and Sunrise and Dvorak's The Golden Spinning Wheel - aren't great but warrant periodic hearings. Dvorak's episodic folk-tale tone poem - it feels almost like the Gray Line Tour of his opera Rusalka - had a sense of sonority that sent signals to the listener heralding turning points in the piece's architecture, creating a parallel narrative to the programmatic events. Interestingly, Gilbert brought a leaner New York sound, though the Philadelphia lushness asserted itself in key moments.

The program is repeated at 8 p.m. Saturday at the Kimmel Center. Information: www.philorch.org or 215-893-1999.