As a guest artist Barker has appeared at De Nederlandse Opera (Cio-Cio San, Emilia Marty), the Royal Opera, London (Jenifer in "The Midsummer Marriage"), the Scottish Opera (Adina in "L'elisir d'amore", Annius in "La clemenza di Tito", and Tatyana), the Vlaamse Opera ("Suor Angelica", Liù in "Turandot"), and the Welsh National Opera (Káťa Kabanová). In 2004 she portrayed Sarah Miles in the world premiere of Jake Heggie's "The End of the Affair" at the Houston Grand Opera with her husband Peter Coleman-Wright as her role-husband and Teddy Tahu Rhodes as her lover. In 2009 she sang Cio-Cio San for her first appearance with the Paris Opera.