Understanding of the specific features of formation of the hydrogen-containing clouds inside of confined spaces is important for fire or explosion protection of the hydrogen facilities. Different sub- sonic modes of light gas (hydrogen and helium) release (with flowrates up to 0.1 g/sec) have been studied experimentally in sealed cylindrical vessel and in cubic enclosure with non-zero gas-tightness. During analysis of the experimental records, two different modes of hydrogen field evolution have been revealed. A criterion for differentiation between stratification and quasi-homogeneous mixing modes are proposed and justified on the base of obtained new experimental data and extension of the known analytical criteria.