The EC funded Naturalhy project is investigating the possibility of promoting the swift introduction ofhydrogen as a fuel, by mixing hydrogen with natural gas and transporting this mixture by means of theexisting natural gas pipeline system to end-users. Hydrogen may then be extracted for use inhydrogen fuel cell applications or the mixture may be used directly in conventional gas-firedequipment. This means that domestic customers would receive a natural gas (methane)/hydrogenmixture delivered to the home. As the characteristics of hydrogen are different from natural gas, theremay be an increased risk to end-users in the event of an accidental release of gas from internal pipework or appliances. Consequently, part of the Naturalhy project is aimed at assessing the potentialimplications on the safety of the public, which includes end-users in their homes. In order tounderstand the nature of any gas accumulation which may form and identify the controllingparameters, a series of large scale experiments have been performed to study gas accumulations withina 3 m by 3 m by 2.3 m ventilated enclosure representing a domestic room. Gas was released verticallyupwards at a pressure typical of that experienced in a domestic environment from hole sizesrepresentative of leaks and breaks in pipe work. The released gas composition was varied andincluded methane and a range of methane/hydrogen mixtures containing up to 50%2hydrogen. Duringthe experiments gas concentrations throughout the enclosure and the external wind conditions weremonitored with time. The experimental data is presented. Analysis of the data and predictions using amodel developed to interpret the experimental data show that both buoyancy and wind drivenventilation are important.