This paper reports experimental results from a series of experiments in which gaseous hydrogen was released into a 31 m3 enclosure and the hydrogen concentrations at a number of points within the enclosure were monitored to assess whether hydrogen accumulation occurred and whether a homogeneous or stratified mixture was formed. The enclosure was located in the open air and therefore subject to realistic, and therefore variable, wind conditions. The hydrogen release rate and the passive vent arrangements were varied. The experiments were carried out as part of the EU Hyindoor Project.