A new approach for consideration of the self-heating modes during monomolecular exothermic reaction has been proposed. The analysis of reactions in the phase plane heating rate and temperature enables detection of the variety of modes of the heating processes and allows consideration of two dimensional parametric model: Todes criterion and Semenov criterion. This gives the possibility of determining the characteristic regions in the corresponding parametric plane. The new thermokinetic parameters were introduced for the phase trajectories analysis. The characteristic values of parameters (and characteristic temperatures) can be determined at the intersection points of the phase trajectories and boundaries of regions. The analytical expression for the thermal explosion critical conditions, which includes Todes criterion, and the new thermokinetic parameters were obtained for the first-order reactions. The condition of applicability of the classical thermal explosion theory was proposed. It was shown that the classical theory of the thermal explosion is the partial case of the presented model.