Computational Fluid Dynamics (CFD) is employed to investigate the hydrogen jet exiting through different shapes of orifices. The effect of orifice geometry on the structure, development and dispersion of a highly under-expanded hydrogen jet close to the exit is numerically investigated. Various shapes of orifices are evaluated, including holes with constant areas such as elliptical and circular openings, as well as, enlarging circular orifices. A three-dimensional in-house parallel code is exploited to simulate the flow using an unstructured tetrahedral finite volume Euler solver. The numerical simulations indicate that, for a high pressure reservoir hydrogen release, the area of the orifice is the main geometric parameter influencing the centerline pressure at the hydrogen-air interface and the transient peak temperature, while the elliptical or expanding orifices slightly mitigate the auto-ignition risks associated with the accidental release of hydrogen. Therefore, circular openings represent the most conservative geometry for the study of auto-ignition of hydrogen. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.