The results of large scale experiments to study gas accumulation within a ventilated enclosure representing a domestic room are presented. Gas was released vertically upwards at a pressure typical of that experienced in a domestic environment from hole sizes representative of leaks and breaks in pipes. The released gas composition was either methane or a methane/hydrogen mixture containing up to 50% hydrogen. During the experiments, gas concentrations throughout the enclosure and the external wind conditions were monitored. A mathematical model has also been developed to describe the gas release as it mixes with air and forms a layer of gas/air mixture in the upper part of the enclosure. The model accounts for both wind and buoyancy driven ventilation, which arises as a result of the formation of the gas accumulation within the enclosure. The results show the importance of buoyancy driven ventilation on the steady state gas concentrations achieved. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.