For 20 years mezzo-soprano Jan DeGaetani and pianist Gilbert Kalish have been performing song recitals of enormous sensitivity and sophistication. By now, in their special repertory--with its strong emphasis on American music--they are unique. One doubts that there is anyone else now on the concert scene who could communicate six Charles Ives songs with the authority that they showed last night at the Library of Congress.

And the other American work on the program, George Crumb's grim "Apparition," was one of many important American works they have commissioned.

They handle this kind of music with the subtlety that one used to hear in Hugo Wolf from Schwarzkopf and Gerald Moore.

Perhaps the most remarkable moment came in a little Ives work called "Down East," with its foggy, harmonically ambivalent introduction, "Songs! Visions of my homeland . . . " and its folkish main verse, "Way down east in a village by the sea," that includes a quote from "Nearer My God to Thee." Such songs, built like patchwork quilts, tend to fall apart. But with DeGaetani and Kalish it was as seamless as a Schubert lied. They do this with exquisite attentions to sonorities and balances.

These same qualities were more spectacularly displayed in the Crumb cycle of six verses from Walt Whitman. This is the poem, born of the Civil War, in which the apparition of death is faced, with the haunting line, "When lilacs last in the dooryard bloomed . . . " As Crumb sets it, the poem is more a single elegiac unit than a sequence of six contrasting verses. Crumb's extraordinary tonal experiments with the piano that have Kalish on his feet playing directly on the strings much of the time show the kind of acute sensivity to pure sound that one expects of Debussy. It is as if the work were an example of impressionism to the nth degree. With its evocative piano interludes and its haunting little vocalises (wordless songs) "Apparition" comes off more like a tone poem than a conventional song cycle, ending with the soprano mouthing soundlessly the words "nesting close to thee."

The recital opened with Haydn's noble arias on the Ariadne at Naxos theme, sung with real rhythmic and harmonic rigor. There were also four delectable Henri Duparc songs to add warmth to the program.

The encore was that wonderful spiritual, "In the Morning When I rise, Give Me Jesus." Sopranos who are white tend to avoid these works for fear of competition with the Leontyne Prices and Jessye Normans of the world. DeGaetani obviously has no such fear and there is no reason that she should. The song positively glowed in her performance.