INERIS has set up large-scale fully instrumented experiments to study the formation of flammable clouds resulting from a finite duration leakage of hydrogen in a quiescent room (80 m(3) chamber). Concentration, temperature and mass flow measurements were monitored during the release period and several hours after. Experiments were carried out for mass flow rates ranging from 0.2 g/s to 1 g/s. The instrumentation allowed the observation and quantification of rich hydrogen layers stratification effects. This paper presents both the experimental facility and the test results. These experimental results have been used to assess and benchmark CFD tools capabilities [1]. (C) 2010 Professer T.Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.