Abstract Deflagrations of hydrogen mixed with air have been studied in an open space and inside a shock tube to provide fundamental data needed for safety evaluations and validation of computer models. The open space tests were performed in 5.2- and 37-m 3 rectangular tents and in a 300-m 3 hemispherical tent that were filled with quiescent, homogenous mixtures ranging from 15 to 57% hydrogen by volume. The mixture was contained by a very thin plastic membrane that was cut just prior to igniting the mixture with a spark at the bottom center to prevent confinement of the mass flow. The information collected included flame front propagation monitored with ionization probes, the pressure–time histories of the resulting blast, and radiated heat obtained from thermal flux sensors. In these experiments the following results were obtained. (i) Deflagration of 30% hydrogen generated a much higher overpressure than deflagration of 9.5% natural gas. (ii) The flame propagation velocity and generated pressure were remarkably influenced by the hydrogen concentration. (iii) Turbulence caused by obstacles within the gas mixture and increasing the gas mixture volume increased the speed of the flame propagation and the overpressure. (iv) The combustion inside a tube also showed a high-speed deflagration. These results are useful to re-examine the existing codes and standards.