An experimental investigation is performed to characterize the detonability of small gaseous clouds with a concentration gradient. Two types of gaseous mixtures are used: (i) a heavy gas C3H8/O-2 (equivalence ratio phi: 0.70 less than or equal to phi less than or equal to 1.66); (ii) a light gas H-2/O-2 (0.50 less than or equal to phi less than or equal to 1.25). The mixtures are initially confined in a hemispherical volume which is characterized by an initial radius R-0 = 0.04 divided by 0.08 m. When the confining is ruptured, the gaseous mixture diffuses into the surrounding air. The concentration distribution is a result of molecular diffusion, gravity and turbulence. Schlieren chronophotographies enable the illustration of the dispersion of the cloud. By means of pressure profiles of blast waves generated by the explosion, the limit between the two explosion phenomena (total and partial explosive charge) is defined. The limit time delay, which leads to a given concentration distribution and for which detonations cannot be observed, is investigated with respect to initial gaseous composition and initial volume of confining. The critical nominal initiation energies in uniform and non-uniform media are characterized.