The highly combustible nature of hydrogen poses a great hazard, creating a number of problems with its safety and handling. As a part of safety studies related to the use of hydrogen in a confined environment, it is extremely important to have a good knowledge of the dispersion mechanism. The release of hydrogen may result in the formation of an explosive atmosphere, which can explode and cause serious damage. The present work investigates the concentration field and flammability envelope from a small scale leak. The hydrogen is released into a 0.47 m x 0.33 m x 0.20 m enclosure designed as a 1/15 – scale model of a room in a nuclear facility. The performed tests evaluates the influence of the initials conditions at the leakage source on the dispersion and mixing characteristics in a confined environment. The role of the leak location and the presence of obstacles, are also analyzed. Throughout the test, during the release and the subsequent diffusion phase, temporal profiles of hydrogen concentration are measured using the thermal conductivity gauges within the enclosure. In addition, the BOS (Background Oriented Schlieren) technique is used to visualise the cloud evolution inside the enclosure. These instruments allow the observation and quantification of the stratification effects.