Numerical simulations have been carried out for spontaneous ignition in the sudden release of pressurized hydrogen into air. A mixture-averaged multi-component approach was used for accurate calculation of molecular transport. Spontaneous ignition and combustion chemistry were accounted for using a 21-step kinetic scheme. To reduce false numerical diffusion, extremely fine meshes were used along with the arbitrary Lagrangian-Eulerian (ALE) method in which convective terms are solved separately from the other terms. Spontaneous ignition of pressurized hydrogen release was previously observed in laboratory tests and suspected as a possible cause of some accidents. The present numerical study has successfully captured this phenomenon and demonstrated a possible mechanism for spontaneous ignition due to molecular diffusion between the shock-heated air and the expanding hydrogen. The role of turbulence in the mixing at the region of the distorted hydrogen-air contact surface and the potential development from the initial laminar flame to a turbulent jet flame has also been discussed. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.