Hydrogen behavior at elevated pressures and temperatures was intensively studied by numerous investigators. Nevertheless, there is a lack of experimental data on hydrogen ignition and combustion at reduced sub-atmospheric pressures. Such conditions are related to the facilities operating under vacuum or sub-atmospheric conditions, for instance like ITER vacuum vessel. Main goal of current work was an experimental evaluation of such fundamental properties of hydrogen air mixtures as flammability limits and laminar flame speed at sub-atmospheric pressures. A spherical explosion chamber with a volume of 8.2 dm(3) was used in the experiments. A pressure method and high-speed camera combined with schlieren system for flame visualization were used in this work. Upper and lower flammability limits and laminar flame velocity have been experimentally evaluated in the range of 4-80% hydrogen in air at initial pressures 25-1000 mbar. An extraction of basic flame properties as Markstein length, overall reaction order and activation energy was done from experimental data on laminar burning velocity. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.