Experimental data from vented explosion tests using lean hydrogen air mixtures with concentrations from 12 to 19% vol. are presented. A 63.7-m(3) chamber was used for the tests with a vent size of either 2.7 or 5.4 m(2). The tests were focused on the effect of hydrogen concentration, ignition location, vent size, and obstacles on the pressure development of a propagating flame in a vented enclosure. The dependence of the maximum pressure generated on the experimental parameters was analyzed. It was confirmed that the pressure maxima are caused by pressure transients controlled by the interplay of the maximum flame area, the burning velocity, and the overpressure generated outside of the chamber by an external explosion. A model proposed earlier to estimate the maximum pressure for each of the main pressure transients was evaluated for the various hydrogen concentrations. The effect of the Lewis number on the vented explosion overpressure is discussed. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.