Numerical simulations have been carried out for spontaneous ignition of pressurized hydrogen releasedirectly into air. Results showed a possible mechanism for spontaneous ignition due to molecular diffusion.To accurately calculate the molecular transport of species, momentum and energy in a multi-componentgaseous mixture, a mixture-averaged multi-component approach was employed in which thermal diffusionis accounted for. To reduce false numerical diffusion, extremely fine meshes were used along with theALE (Arbitrary Lagrangian-Eulerian) method. The ALE method was employed to track the movingcontact surface with moving clustered grids. A detailed kinetic scheme with 21 elementary steps and 8reactive chemical species was implemented for combustion chemistry. The scheme gives dueconsideration to third body reactions and reaction-rate pressure-dependant ?fall-off? behavior. The autoignition of pressurized hydrogen release was previously observed in laboratory tests [2-3] andsuspected as possible cause of some accidents. The present numerical study successfully captured thisscenario. Autoignition was predicted to first take place at the tip region of the hydrogen-air contact surfacedue to mass and energy exchange between low temperature hydrogen and shock-heated air at the contactsurface through molecular diffusion. The initial flame thickness is extremely thin due to the limitingmolecular diffusion. The combustion region extends downward along the contact surface as it movesdownstream. As the hydrogen jet developed downstream, the front contact surface tends to be distorted bythe developed flow of the air. Turbulence plays an important role in mixing at the region of the distortedcontact surface. This is thought to be a major factor for the initial laminar flame to turn into a final stableturbulent flame. Keywords: Spontaneous Ignition, Diffusion Ignition, Shock, Mach Disk, Molecular Transport, and ALE.