A new method for theoretical analysis of the self-heating kinetics for monomolecular exothermic reactions has been proposed. It has been shown, that consideration of the phase trajectories of reaction in the phase plane: the heating rate - temperature enables detection of the qualitative changes of the phase portrait under the changes of the Todes or Semenov criteria. This gives a possibility to analyze the diversity of various reaction modes for any reaction order. From this point of view, the classical Semenov theory of thermal explosion developed for zero-order reactions is a particular case. As an illustration of this method, the detailed phase trajectories analysis has been performed for the case of first-order reactions. The regions of the thermal explosion degeneration, fastest reaction mode region and transition regions have been established. The necessary and sufficient conditions for the thermal explosion have been formulated. Examples of the application of the method for calculation of specific reactions are presented; the comparison with the classical theory is performed. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.