Report presents the preliminary experimental results on hydrogen subsonic leakage in a closed vesselunder the well-controlled boundary/initial conditions. Formation of hydrogen-air gas mixture cloudwas studied for a transient (10 min), upward hydrogen leakage, which was followed by subsequentevolution (15 min) of explosive cloud. Low-intensity ( m31046.0 ?? 3/sec) hydrogen release wasperformed via circular (diameter 0.014 m) orifice located in the bottom part of a horizontal cylindricalvessel ( 4 m 3). A spatially distributed net of the 24 hydrogen sensors and 24 temperature sensors wasused to permanently track the time dependence of the hydrogen concentration and temperature fieldsin vessel. Analysis of the simultaneous experimental records for the different spatial points permits todelineate the basic flow patterns and stages of hydrogen subsonic release in closed vessel in contrast tohydrogen jet release in open environment. The quantitative data were obtained for the averaged speedsof explosive cloud envelop (50%2fraction of the Lower Flammability Limit (LFL)) propagation in thevertical and horizontal directions. The obtained data will be used as an experimental basis fordevelopment of the guidelines for an indoors allocation of the hydrogen sensors. Data can be also usedas a new benchmark case for the reactive Computational Fluid Dynamics codes validation.