Among the factors affecting explosion venting, ignition position deserves more detailed investigation because it significantly affects the propagation of flame and the relief of internal pressure during a vented explosion. Explosion venting of hydrogen air mixtures with equivalence ratio ranging from 0.6 to 5.0 was experimentally investigated in the cases of back, central and front ignition. The results show that front ignition always leads to the minimum internal and external overpressure. Central ignition results in the maximum internal overpressure for all equivalence ratios and back ignition produces the maximum external overpressure for hydrogen-rich mixtures. Both the maximum internal and external overpressures first increase and then decrease with the increase of hydrogen equivalence ratio. The evolution of the external flame includes the formation of a fireball and a jet flame. The maximum lengths of the external flame are nearly identical when hydrogen air mixtures are back and centrally ignited, which are larger than those in the front ignition case. For equivalence ratio of 3.0 and 5.0, the interaction of internal flame with the backward propagating pressure wave resulted from external explosion leads to the appearance of fine cellular structure and the oscillation of the internal pressure. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.