We present an experimental investigation of the different concentration build-up regimes encountered during a release of helium/air mixture in an empty enclosure without ventilation. The release is a vertical jet issuing from a nozzle located near the floor. The nozzle diameter, the flow rate and the composition of the injected mixture have been varied such that the injection Richardson number ranges from 6 x 10(-6) to 190. The volume Richardson number, which gives the ability of the release to mix the enclosure content, ranges from 2 x 10(-3) to 2 x 10(4). This wide range allowed reaching three distinct regimes: stratified, stratified with a homogeneous upper layer and homogenous. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.