Two methods of oxidant-fuel gas mixture preparation at high pressure have been analyzed and fit with the objective to avoid explosion damage during filling of cylinder. First method is called "safe method" as the flammability range is not crossed during cylinder filling due to the introduction of major component first in cylinder. The second method is called "accurate method" as minor component is introduced first in cylinder leading to a risk of explosion during the introduction of major component at the end while crossing the flammability domain. The diagrams representing the maximum filling pressure of filling to prevent any accident are given as a function of fuel concentration for lean and rich mixtures. These diagrams are deduced from flammability and detonation limits versus pressure at ambient temperature. Both methods have been applied to hydrogen air mixtures. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.