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 10-6to 190. The volume Richardson number, which gives the ability of the release to mix the enclosure content, ranges from 2 10-3to 2 104. This wide range allowed reaching three distinct regimes: stratified, stratified with a homogeneousupper layer and homogenous.