We present an experimental study on the dispersion of helium in an enclosure of 1m3 with natural ventilation through one vent. Three vent geometries have been studied. Injection parameters have been varied so that the injection Richardson number ranges from 2 10-6 to 9 and the volume Richardson number, which gives the ability of the release to mix the enclosure content ranges from 8 10-4 to 900. It is found that the vertical distribution of helium volume fraction can exhibit significant gradient. Nevertheless, the results are compared to the simple analytical model based on the homogenous mixture hypothesis which gives fairly good estimate of the maximum helium volume fraction.