In this article the steady-state process leading to ignition of a combustible mixture of hydrogen, oxygen, and nitrogen by a hot flat plate in a boundary layer flow is studied, including the effect of thermal diffusion. For sufficiently large values of the plate temperature, the ignition event corresponds to a typical chain-branching explosion with negligible heat release, in a first approximation. In the framework of the reduced kinetic mechanism appropriate for this regime, the boundary layer equations are solved by using the fact that the chain branching reaction H + O z ~ OH + O has a relatively large activation energy. Assuming a three-layer structure, the governing equations reduce to a single integrodifferential equation for the concentration of atomic hydrogen. Close to the plate leading edge, the concentration of atomic hydrogen around the plate surface is higher when thermal diffusion is included, but, later on, this effect enhances the evacuation of atomic hydrogen by the external convective/diffusive layer. Thus, in spite of the initial enrichment in molecular hydrogen by thermal diffusion, the final ignition distance is shown to be enlarged by a factor of about 2, under usual conditions. On the other hand, for low plate temperature ignition, thermal diffusion produces the expected reduction in the ignition distance.