We experimentally investigated the pressure buildup and flame behavior during the vented deflagration of hydrogen-air mixtures with concentrations ranging from 13% to 39% that were centrally ignited in a 1-m(3) rectangular vessel with a 500 mm x 400 mm top vent. The performance of some available models for estimating the maximum reduced overpressure was experimentally evaluated. The maximum reduced overpressure increased from approximately 3 kPa to 100 kPa as hydrogen concentration increased from 13 to 39%. Turbulent pressure oscillations with frequencies of 200-300 Hz triggered by external explosions were observed in our tests with 22-39% hydrogen-in-air mixtures. Molkov's best fit and conservative models predict the maximum reduced overpressure well for lean and rich hydrogen mixtures, respectively. The average speed of the external flame first decreases to a minimum value with the fireball expanding to its maximum size and then increases. As hydrogen concentration increases, the maximum length and duration of the external flame increases 'and decreases, respectively. 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.