Posh diva though she is, Renée Fleming has, it would seem, lost none of her ability to take you by surprise. The first half of her latest Barbican recital closed with George Crumb's Apparition, a song cycle on texts by Walt Whitman, composed in 1979 for Fleming's teacher Jan DeGaetani.

It's surprising to realise that Fleming learned her formidable, all-embracing artistry from one of the most distinguished post-war exponents of contemporary music. It also has to be said that few singers of Fleming's popularity would risk including a work like Apparition in their programmes.

The cycle deals with the nature of mortality and envisions death as the ecstatic consummation of human existence. Fleming tackled it with frightening fervour, rapturously greeting the "strong deliveress" who would sweep her away in "the flood of bliss," and seductively begging death to "undulate round the world, arriving, arriving."

The span of the vocal line is immense, and she negotiated it with jaw-dropping beauty. Pianist Hartmut Höll surrounded her with weird shivers and tintinnabulations that sounded like the toil of earthly life receding into the distance. This was a great achievement, though the rest of the recital was perhaps more uneven.

Fleming opened with a group of songs by Purcell, which sounded appealingly spontaneous, but also shapeless. The Altenberg Lieder, Berg's scary study of modernist dissolution, came in the second half, finely judged in its combination of intense declamation and electrifying high notes. Höll, however, used a piano reduction of Berg's startling orchestral score, which blunted its impact.

There was some glorious Schumann at the close, though her inclusion, earlier in the evening, of André Previn's inconsequential The Giraffes Go to Hamburg was something of a puzzle: Fleming's uniqueness derives from her ability to sing almost anything, though every so often one ends up questioning her artistic judgment.