Explosion relief panels or doors are often used in industrial buildings to reduce damages caused by gas explosions. Decades of research have contributed to the understanding of the phenomena involved in gas explosions in order to establish an effective method to predict reliably the explosion overpressure. All the methods predict a monotonic increase of the overpressure with the concentration of the gas in the range from the lower explosion limit to the stoichiometric one. Nevertheless, in few cases, a non-monotonic behaviour of the maximum developed pressure as a function of hydrogen concentration was reported in the literature. The non-monotonic behaviour was also observed during experimental tests performed at the Scalbatraio laboratory at the University of Pisa, in a 25 m(3) vented combustion test facility, with a vent area of 1,12 m(2). This paper presents the results obtained during the tests and investigates the possible explanations of the phenomena. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.