29 batches of vented explosion tests were conducted in a 12 m(3) concrete chamber filled with methaneair mixtures to investigate the effects of methane concentration and venting pressure on the development of overpressure inside the chamber. The deflagrations were vented from a square side window with a venting area of 0.64 m(2) upon rupture of the vent cover. The venting pressures were varied by using six different types of vent covers, and determined by performing a numerical simulation. Methane concentrations in the mixed gas varied between 6.5 and 13.5 vol.%, covering both lean and rich combustion regimes. The generation conditions of the four types of overpressure-time profiles with different overpressure transients were summarized. Among the overpressure transients, the Delta P-1 caused by failure of the vent cover and the Delta P-4 resulted from the coupling between acoustic mode and flame were basically dominant. The rate of Delta P-1 rise as well as peak value of Delta P-1 and Delta P-4 showed a same trend of first increasing and then decreasing with the methane concentration from lean to rich. They reached their maximum value at the methane concentration of about 9.5%, whereas the rate of Delta P-4 rise was found to be insensitive to the methane concentration. The peak value of Delta P-1 increased with the venting pressure, while that of Delta P-4 first increased and then decreased as the venting pressure increased. In addition, the rates of Delta P-1 and Delta P-4 rise were both insensitive to the venting pressure. The occurrence of Delta P-4 was depended on the methane concentration and the venting pressure. The differences in the behavior of these overpressure transients suggest their different generation mechanisms. (C) 2016 Elsevier Ltd. All rights reserved.