Flame acceleration (FA) and explosion of hydrogen/air mixtures remain key issues for severe accident management in nuclear power plants. Empirical criteria were developed in the early 2000s by Dorofeev and colleagues, providing effective tools to discern possible FA or DDT (Deflagration-to-Detonation Transition) scenarios. A large experimental database, composed mainly of middle-scale experiments in obstacle-laden ducts at atmospheric pressure condition, has been used to validate these criteria. However, during a severe accident, the high release rate of steam and non-condensable gases into the containment can result in pressure increase up to 5 bar abs. In the present work, the influence of the unburnt gas initial pressure on flame propagation mechanisms was experimentally investigated. Premixed hydrogen/air mixtures with hydrogen concentration close to 11% and 15% were considered. From the literature, we know that these flames are supposed to accelerate up to Chapman-Jouguet deflagration velocity in long obstacle-laden tubes at initial atmospheric conditions. Varying the pressure in the fresh gas in the range 0.6-4 bar, no effects on the flame acceleration phase were observed. However, as the initial pressure was increased, we observed a decrease in the flame velocity close to the end of the tube. The pressure increase due to the combustion reaction was found to be proportional to the initial pressure according to adiabatic isochoric complete combustion. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.