In the present work, the analytical investigation of the phase trajectories structures on the plane: heating rate-temperature for bimolecular exothermic reactions was carried out. This gave a possibility to divide the parametric plane Todes parameter-Semenov parameter into five characteristic regions with various modes of self-heating. It was established that all regions converge at a single point which determines the thermal explosion degeneration conditions. The correlation between Todes parameter, Semenov parameter and concentration ratio of initial reactants at the limit of the thermal explosion degeneration was found. The critical conditions for thermal explosion at any ratio of concentrations of initial reactants were determined. The approach proposed is a general one and allows analyzing both first and second order reactions. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.