Thais/Mazepa, Grange Park / Wales Millennium Centre, Alresford/Cardiff

It's good to see Grange Park, now in its ninth season, turn to Massenet. The French composer (1842-1912), whom Debussy ridiculed but secretly stole from, writes the ideal kind of music for Grange Park aficionados.

Thaïs (1894) is best known for the famous violin "Méditation", which wells up at several points in the opera. Some think it finer than Massenet's calling-card, Manon. They may be right.

Louis Gallet's libretto is not problem-free. The tale of St Thaïs, whore-turned-saint, was retailed by the German poetess Hroswitha in the 10th century, and reworked by Massenet's contemporary, Anatole France (1844-1924). Here, Thaïs progresses from fallen woman to saint in scarcely three weeks (just 20 minutes, actually). The opera's seven scenes gloss over her semi-saintly childhood; yet we need the backgrounds of both principals to understand their mutually reflecting ascent to apotheosis and lapse into depravity.

Brother Athanaël, who sanctimoniusly struggles to purify Thaïs while inwardly lusting for her (shades of The Hunchback of Notre Dame), has enjoyed a cosseted upbringing in 4th-century Alexandria, whose excess, epitomised by the philosopher-libertine Nicias (the sleazily sexy Jeffrey Lloyd Roberts) and his Dionysiac retinue, the reclusive monks despise and eschew.

For Thaïs's real depth - an in-depth study idea of two complex characters embarked on almost diametrically opposite paths - we have to scour Massenet's music. The paradoxes, the inner tensions between saint and sinner, of volition and denial, are explored in a welter of well-worked post-Wagnerian motifs.

Athanaël is sung by the baritone Ashley Holland, who brings considerable warmth to Massenet's vocal lines even while his on-stage presence remains awkward. Thaïs's big solo in Act II counts among 19th-century opera's great scenas; and their ensuing twosome (Thaïs offers glorious opportunities for soprano-baritone duets) is top-drawer stuff.

The conductor, Martin Andre, brings experience and sensitivity to the orchestral score. He knows his Massenet, and relishes it. The cellos, in particular, were out of this world.

Anne-Sophie Duprels (Thaïs) emerges as a capable actor, especially in the crucial, bedraggled oasis scene (Massenet tacked it on for the opera's revival, with Mary Garden, in 1898). Her robust and resilient singing wrenched hearts. The audience, rightly, liked the monk Palémon (the impressive South African bass Vuyani Mlinde, making his British debut), who wisely denigrates his friend's proselytising zeal.

David Fielding's direction seemed initially thin on cogent ideas, but his designs (a huge cross assuming many guises; a glaring on-screen lava flow suggesting blistering desert; ghoulish, mummified make-up for Thaïs at the oasis; and a beautiful, sepulchral sculpture for the distantly viewed dying scene) were masterly. Make sure you stick it out to the end.

If you have the courage to risk the relatively unknown, try to catch Welsh National Opera's impressive new staging of Tchaikovsky's Mazepa when it tours to other venues. WNO's planned series of 'Russian' seasons is developing apace.

Tchaikovsky composed Mazepa (1884) midway between Eugene Onegin and The Queen of Spades. Its libretto, also based on Pushkin, with input from the composer, is striking and forceful.

Directed by the team of Christian Fenouillat and Moshe Leiser, the cast make a splendid fist of this too rarely seen opera. And you have only to listen to WNO's orchestra under Alexander Polianichko playing the Act III Prelude, a blistering "Battle of Poltava" episode, to realise that Cardiff offers a fine orchestra as well as its world-class chorus.

'Thaïs' to 30 June; festival to 4 July; (01962 868 888; www.grangeparkopera.co.uk); 'Mazepa' touring every Saturday to 15 July (029 2063 5000; www.wno.org.uk)