The EC funded Naturalhy project is investigating the possibility of promoting the swift introduction of hydrogen as a fuel, by mixing hydrogen with natural gas and transporting this mixture by means of the existing natural gas pipeline system to end-users. Hydrogen may then be extracted for use in hydrogen fuel cell applications or the mixture may be used directly in conventional gas-fired equipment. This means that domestic customers would receive a natural gas (methane)/hydrogen mixture delivered to the home. As the characteristics of hydrogen are different from natural gas, there may be an increased risk to end-users in the event of an accidental release of gas from internal pipe work or appliances. Consequently, part of the Naturalhy project is aimed at assessing the potential implications on the safety of the public, which includes end-users in their homes. In order to understand the nature of any gas accumulation which may form and identify the controlling parameters, a series of large scale experiments have been performed to study gas accumulations within a 3 m by 3 m by 2.3 m ventilated enclosure representing a domestic room. 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 pipe work. The released gas composition was varied and included methane and a range of ethane/hydrogen mixtures containing up to 50%2hydrogen. During the experiments gas concentrations throughout the enclosure and the external wind conditions were monitored with time. The experimental data is presented. Analysis of the data and predictions using a model developed to interpret the experimental data show that both buoyancy and wind driven ventilation are important.