Certain of the ensembles presented in Portland by Friends of Chamber Music, such as the Takacs and Emerson Quartets, become audience favorites and return frequently. The Pacifica Quartet, which made its debut here in April of last year and returned Monday night at First Baptist Church, promises to become one of them. We should be so lucky.

The four -- violinists Simin Ganatra and Sibbi Bernhardsson, violist Masumi Per Rostad and cellist Brandon Vamos -- are among the most accomplished of the younger generation of chamber players. Currently in residence at the University of Illinois at Urbana-Champaign, they've been playing together for 14 years, in the process winning most of the country's top chamber music awards.

Given that they've recently toured the complete Beethoven cycle and recorded all of Felix Mendelssohn's quartets, it was no surprise that their performances of Mendelssohn's Op. 13 and Beethoven's Op. 132 were thoughtful and polished to a high degree. The juxtaposition was interesting in itself: The 18-year-old Mendelssohn wrote his early A minor with Beethoven's late quartets in mind, and his mastery of the form was awe-inspiring. It was just as well that his was played first, however, because as lovely as it was, it suffered in comparison, with none of Beethoven's sense of struggle.

The Beethoven was a show-stopper. Its outer movements were generally up-tempo and richly colored, with tight ensemble maintaining that crucial quartet balance between individual players and the whole.

The central "Heiliger Dankgesang," or "Holy Song of Thanksgiving" (Beethoven wrote it after recuperating from severe illness) was exquisite: Minimal vibrato enhanced its antique modal feel, and the pacing was perfectly measured for maximum excitement as Beethoven held out the cadence for what always seems like forever no matter how many times you hear the piece. I didn't know whether to fall to my knees and weep or jump out of my seat.

Between the two was Gyorgi Ligeti's String Quartet No. 1, an absolute delight that doesn't get played nearly enough. Essentially an elaboration of a four-note motif through a dozen short movements played without pause, it was rich in pungent harmonies reminiscent of Béla Bartók, with colors drawn from wide-ranging string techniques, from glassy muted stroking to violent pizzicatos snapping off the fingerboard. The daring performance projected the wit and humanity that makes Ligeti so great even at his most abstract and atonal.

There was little to complain about. Bernhardsson was sometimes lost behind Ganatra's gorgeous, ringing tone in the Mendelssohn, but that was partly a factor of the writing and partly of the room (which was, incidentally, very warm and stuffy). The acoustic was punishing for a small string ensemble, magnifying any flaw and leaving slowly-bowed pianissimos sounding scratchy. Pity the lesser quartet that has to play here.