Experiments on explosion venting of a stoichiometric hydrogen-air mixture ignited near the top vent of a 1-m(3) rectangular vessel with a hinged aluminum vent panel were performed to investigate the effect of the panel area density on the pressure build-up and flame behavior. When using aluminum panels, three pressure peaks could be distinguished in the pressure-time histories. The first pressure peak, which increases with the panel area density, is the dominant one. However, the second and the third pressure peaks, with magnitudes ranging from 5 to 10 kPa, are independent of the panel area density. The use of aluminum panels weakens the external explosion because the gas mixtures were vented laterally shortly after the vent panel was opened. Panel inertia has a negligible effect on the final stage of the downward propagating flame. The maximum external flame length decreases with the increase in panel area density. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.