Flame acceleration (FA) and explosion of hydrogen/air mixtures remain key issues for severe accidentmanagement in nuclear power plants. Empirical criteria were developed in the early 2000s byDorofeev and colleagues, providing effective tools to discern possible FA or DDT (Deflagration-to-Detonation Transition) scenarios. A large experimental database, composed mainly of middle-scaleexperiments in obstacle-laden ducts at atmospheric pressure condition, has been used to validate thesecriteria. However, during a severe accident, the high release rate of steam and non-condensable gasesinto the containment can result in pressure increase up to 5 bar abs. In the present work, the influenceof the unburnt gas initial pressure on flame propagation mechanisms was experimentally investigated.Premixed hydrogen/air mixtures with2 H et61539; close to 11percent were considered. From the literature we knowthat these flames are supposed to accelerate up to Chapman-Jouguet deflagration velocity in longobstacle-laden tubes at initial atmospheric conditions. Varying the pressure in the fresh gas in therange 0.6-4 bar, no relevant effects on the flame acceleration phase were observed. However, as theinitial pressure was increased, we observed a decrease in the flame velocity close to the end of thetube. The pressure increase due to the combustion reaction was found to be proportional to 0 p .