The occasion to hear the complete "Italienisches Liederbuch" by Hugo Wolf is a rare one. It is rarer still to hear as ravishing a performance as Sunday's at the Phillips Collection by Donald Collup and Susan Tilton, with Walter Huff at the piano.

The "Italian Songbook" is a work of elusive beauty, perhaps the most ambitious of Wolf's cycles. There are few climaxes; its passion remains restrained. And in its 46 songs the mood suggests the brief intimacy of conversations between lovers and old friends. Wolf was pleased with its subtle, simple beauty, and thought it the most original of his works.

The composer's love was understandable in Donald Collup's voice. Here is a young baritone of truly remarkable natural gifts and unquestionable musical wisdom. His lyric instrument is even and rich throughout the range, and his meticulous attention to the text made the music seem more natural. He whispered sensuously, disarmingly in "Und willst du deined Liebsten sterben schen," and he could take aside the whole audience with the conspiratorial tone of his "Geselle, woll'n wir uns in Kutten hullen." Collup's artistry recalled names like Souzay and Fischer-Dieskau, but his distinctive timbre and presence already make him a unique treasure.

Tilton was fine as well. She is a sweet soprano with angelic top notes and almost childish innocence below the staff. Her voice is steady and always lovely, yet distant in feeling, relaxed in phrasing, and not yet of the right temperament to bring Wolf's music to life. The humor of "Wie lange schon war immer Verlangen," for instance, was better communicated by Huff's piano than by Tilton's singing.

Throughout the concert the piano served the poetry as well as the voice, and the music was irresistible. In all, it was a gorgeous concert. The entire songbook will be broadcast on WETA Sunday at 10 p.m.