Abstract With a freely expanding unwrinkled spherical flame as an example, it is shown that deflagration-to-detonation transition in unconfined gaseous systems is possible, provided the flame is close to its propagability threshold and the deficient reactant Lewis number is not too small. The transition is triggered by the thermal runaway induced by positive feedback between the accelerating flame and the flame-driven pressure build-up. A possible way to account for the impact of wrinkling is discussed.