A combustor for lean mixtures, employing heat recirculation and a new gas burner generating a swirling central flame, was developed and tested. Stability curves for burning lean mixtures of methane/air were determined. The leanest mixture which was burned before extinction, at a preheat temperature of 540° c and a flow rate of 1000 × 10−6m3/s, contained 3.2% of methane. This figure is 60% of the normal flammability limit. The stability curves were determined at mean preheat temperatures of 35, 460 and 555° c, and for burners with some different geometrical parameters. Special attention has been paid to explore the effect of entrainment of uncontrolled amounts of air on the stability curves.