made her debut at the Paris Opera in 1978, as Marguerite in Faust, next playing Fortune and Drusilla in Monteverdi's 1642 work, The Coronation of Poppea the same season. She later sang Constanza in The Seraglio and Gilda in Rigoletto. She also continued to have success in Mozart roles, including in Così fan tutte and Idomeneo. She sang both Susanna and the Countess in The Marriage of Figaro, appearing at the Prague National Theatre in 1993 as the Countess. She performed for two seasons at Geneva, including as Gilda in 1981 and Mireille later that year. Masterson was a member of the English National Opera company that toured five cities in the U.S. in May and June 1984, singing Gilda. She also sang roles in The Merry Widow and Lucia di Lammermoor, among others. Masterson played a significant part in the reintroduction of Handel's operas to the mainstream repertoire. Her purity of line and facility for ornamentation, coupled with excellent diction, helped to bring to life his works which, as recently as the 1960s, were considered unperformable. In 1983, Masterson won a Laurence Olivier Award for her performance in the title role of Semele at Covent Garden. Her other Handel roles included Romilda in Xerxes for ENO, Cleopatra in Giulio Cesare opposite Janet Baker for ENO and later for Houston Grand Opera and San Francisco Opera, Berenice in Scipio for the Handel Opera Society at Sadler's Wells, and Morgana in Alcina at Aix en Provence. On the concert platform she sang the Queen of Sheba in Solomon. Some of her greatest successes were in the French repertoire. Of her 1974 Manon for ENO, The Times commented, "The Coliseum has found a worthy successor to Elizabeth Harwood in the delectable form of Valerie Masterson, who has here her greatest success to date with the company. She looks at once charming, fragile and seductive, and she sings with light insouciance and easy forward tone." Her Marguerite in Gounod's Faust in Paris and London drew warm notices: "The jewel song has the girlish delight that Gounod wanted (but does not always get) delivered in the easy coloratura Miss Masterson first displayed in The Pirates of Penzance." She sang Juliet, in Romeo and Juliet, in London and internationally, Leila in The Pearl Fishers, and the title role in Louise. As a home-grown British soprano with a charming personality and attractive appearance, Masterson became popular