The use of hydrogen as an energy carrier for the future is conditioned by its safety. Hydrogen is commonly and incorrectly perceived as being a more dangerous gas than methane, since the latter is widely used and thus considered to be acceptable. The paper analyses deflagrations of H-2/air and CH4/air mixtures at low concentrations (close to the lower flammability limits) and, in particular, focuses on the phenomenology and dangerous aspects of this kind of combustion. The related possible accidents involve closed environments (garages, laboratories, service rooms, internal volumes of buses and cars, etc.) where ignition sources are present. In these cases, combustion probably takes place as soon as the fuel concentration reaches the lower flammability limit. Hydrogen and methane are compared on the basis of (1) their general energetic characteristics, (2) a theoretical examination of the main parameters related to the combustion phenomenon, and (3) the experimental results of the LargeView2 apparatus, where more than 300 deflagration tests were made with both gases in a vented multi-compartment container. (c) 2004 Elsevier Ltd. All rights reserved.