An experimental investigation on the explosion characteristics of hydrogen-air mixtures diluted with inert gases (Ar, N-2, CO2) was carried out in a cylinder chamber at room temperature (298 K) sub-atmospheric pressures (40 kPa, 60 kPa, 80 kPa, 100 kPa). Based on the pressure history recording, some explosion parameters, e.g., maximum explosion pressure (P-max), maximum rate of pressure rise ((dp/dt)(max)), deflagration index (K-G) and combustion duration (theta) were derived. Effects of initial pressure (P-0), equivalence ratio (phi) and dilution fraction (Z%) on these explosion indices were discussed. The results show that with the increase of initial pressure, the maximum explosion pressure, maximum rate of pressure increase and deflagration index increase monotonously while the combustion duration decrease. The diluent effect on these parameters is increased in the order of Ar, N-2 and CO2. Linear correlations P-max/P-0 = f(Z), P-max =f(P-0) and (dp/dt)(max) =f(P-0) were found. In addition, the heat loss to the walls during the explosion propagation was estimated on the basis of the difference between the adiabatic and experimental measured pressure. The energy loss increases with the increase of initial pressure and specific heat of the dilutions. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.