A model calculation of spontaneous ignition in an autoxidative powder bed has shown that the critical state can be classified into Frank-Kamenetskii and oxygen-deficient types. The first type includes an irreversible transient process to ignition at the critical point and its thermal stability is rather frail against the changes in ambient conditions. In the second type, the oxygen diffusion suffocatingly controls the rate of heat generation in the bed and the thermal stability is comparatively robust. The types can be largely discriminated by the material and the porosity of the bed.