Abstract: In the present paper a theoretical description of thermal explosion (spontaneous ignition),which occurs in exothermically reacting systems, in provided. The steady-state criterion for thermal explosion to occur is described. By means of perturbation theory, functional forms are analysed for the most generalized circumstances in which the Biot number (Bi) is arbitrary and in which the reaction rate is generally represented by f (θ).Asymptotic solutions are derived for the limit of large activation energies where the dimensionless activation energy ε is small (i.e.ε0).Corresponding critical values for the Frank-Kamenetskii parameters, reduced reaction rate, δ and reduced central temperature-excess,θ_0 given by using these asymptotic equations, are used for obtaining critical temperatures T_m and T_0,cr,where T_m is critical ignition temperature and T_0,cris the critical (maximum ) central temperature, for some typical samples. These results are provided in Tables 5 and 6.