This work presents a parametric study on the similitude between hydrogen and helium distribution when released in the air by a source located inside of a naturally ventilated enclosure with two vents. Several configurations were experimentally addressed in order to improve knowledge on dispersion. Parameters were chosen to mimic operating conditions of hydrogen energy systems. Thus, the varying parameters of the study were mainly the source diameter, the releasing flow rate, the volume and the geometry of the enclosure. Two different experimental set-ups were used in order to vary the enclosure's height between 1 and 2 m. Experimental results obtained with helium and hydrogen were compared at equivalent flow rates, determined with existing similitude laws. It appears, for the plume release case, that helium can suitably be used for predicting hydrogen dispersion in these operating designs. On the other hand - when the flow turns into a jet - non negligible differences between hydrogen and helium dispersion appear. In this case, helium - used as a direct substitute to hydrogen - will over predict concentrations we would get with hydrogen. Therefore, helium concentration read-outs should be converted to obtain correct predictions for hydrogen. However such a converting law is not available yet. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.