Orchestra of the Age of Enlightenment/ Rattle/Ernman, Barbican, London

Summon your courage and take a running jump...

The best thing about stepping into Magdalena Kozena's pretty shoes at only 24-hours' notice is that there is no time to panic. If Malena Ernman, who did just that in Friday's programme of Mozart arias with the Orchestra of the Age of Enlightenment and Sir Simon Rattle at the Barbican, was nervous, it didn't show. "E amore un ladroncello", Dorabella's excitable post-adultery aria from Cosi fan Tutte, is hardly the easiest piece to warm up with, but aside from Ernman's peculiar blanching of the word "pace", this was a relaxed yet lively performance.

With necessarily scant rehearsal time, it was obvious which works Ernman had previously performed. "Al desio", the version of Susanna's Act II aria from Le Nozze di Figaro in which she suddenly acquires the formality of a Fiordiligi, showed a formidable technique but lacked the poetic fluidity of "Deh vieni". Like "Non piu di fiori" from La clemenza di Tito, it is as much a showcase for the woodwind as for the singer, and OAE's most incisive section did not disappoint. Ernman's "Laudamus Te" from the C minor Mass was blithe and brilliant, as was Cherubino's aria "Non so piu", with each of the libidinous page's bucolic confessors - woods, mountains, flowers, breezes etc - cleverly individuated. Ernman has tremendous charm, and if she is guilty of over-egging details at the expense of line, I can think of another Swedish mezzo who did much the same at an equivalent age and has since become one of the greatest interpreters of words and music.

Throughout the concert, I was impressed by Rattle's sympathetic accompaniment. His reputation is stronger in symphonic repertoire than in opera, yet this was a highly literate performance. The faux-baroque gestures of the ballet music from Idomeneo were no less satisfying, though here, as in Haydn's Oxford Symphony, I was troubled by the gulf between the definition and engagement of the phrasing of the cellos and basses and that of the upper strings. In an age when gut violins have become richer and stronger and warmer, OAE are beginning to sound rather retro.