The study of hydrogen as an alternative fuel, clean and "environment friendly", has been in the last years and continues to be object of many studies, international projects and standard development. Hydrogen is a fundamental energy carrier, to be developed together with other renewable resources, for the transition to a sustainable energy system. But experience has shown how often the introduction and establishment of a new technology does not necessarily pass through radical changes, but can be stimulated by slight modifications to the "present situation". So the worldwide experience with natural gas, as industrial, automotive and domestic fuel, has been the incentive to the present interest towards hydrogen methane mixtures. The possible use of existing pipeline networks for mixtures of natural gas and hydrogen offers a unique and cost-effective opportunity to initiate the progressive introduction of hydrogen as part of the development of a full hydrogen system. The aim of the work presented in this paper is the investigation of the dispersion and stratification properties of hydrogen and methane mixtures. Experimental activities have been carried out in a large scale closed apparatus, characterized by a volume of about 25 m(3), both with and without natural ventilation. Mixtures of 10%vol. hydrogen 90%vol. methane and 30%vol. hydrogen 70%vol. methane have been studied with the help of oxygen sensors and gas chromatography. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.