When vocal/guitar recitals work as naturally as the Isabel Leonard/Sharon Isbin concert presented on Tuesday by the Philadelphia Chamber Music Society at the Kimmel Center, you wonder why they don't happen constantly.

Leonard's mezzo-soprano maintains its richness when scaled back to an equitable balance with Isbin's discreetly amplified guitar. Both are highly cultivated artists who have worked on this program over the last two years. But such circumstances don't come along often - right down to their complementary black gowns.

The atmosphere was informal and chatty, maybe a little too relaxed. Leonard seemed to have great faith in the music's natural charm (which is considerable), with texts firmly integrated into a seamless musical line. But in the folk-ballad adaptations of Federico García Lorca (collected over the poet's brief lifetime and transcribed from his recordings), the narrative needed more help, especially over a series of musically repetitive strophes.

The question is how far to go toward flamenco vocalism. The Leonard/Isbin stance seems to take its cues from the aristocratic elegance heard in 1930 recordings by composer Manuel de Falla and soprano Maria Barrientos. Only at the end of Falla's Seven Popular Spanish Songs did Leonard veer toward the earthier flamenco zone.

This approach made sense, considering that Isbin's technique offers interpretive options, mainly slower tempos than the more typically frenetic folk manner. Her distinctive sustaining power comes not just in her use of sound but in the force of her concentration that gives subtle incandescence even to her softer playing. Coloration is vivid, from solid baritone of the lower strings to treble realms that are too robust to be ethereal but that still offer glimpses of heaven. Isbin's solo moments - including Isaac Albéniz's Asturias and Francisco Tarrega's Recuerdos de la Alhambra - were also feasts of beautifully sculpted phrases, punctuated by nanosecond pauses giving them room to breathe.

Leonard and Isbin were best with Lorca behind them in the first half and songs by Joaqúin Rodrigo, Xavier Montsalvatge, and Falla in the second. The stage patter included the backstory behind Rodrigo's "Aranjuez, ma pensée," which uses music from the second movement of his famous Concierto de Aranjuez with lyrics by his wife, Victoria Kamhi, written after she nearly died from the after-effects of childbirth. Leonard more deftly conveyed the meaning of the words, but during the duo's encores, she took even greater chances with projection, and with such good effect one wished the evening had started at the artistic level where it ended.