In the present paper, the theoretical analysis of the thermal explosion critical conditions for solid state exothermic reactions was conducted. The model proposed is based on the concept of diffusion kinetics of reaction product layer formation (Ginstling-Brownstein law). The dependence of the critical temperatures on the parameters of reacting system was obtained. The parametric boundary of the thermal explosion realization and conditions of its degeneration were defined. The influence of the size of the preformed diffusion zone on the critical conditions was investigated for the solid phase heterostructures. The demarcation lines that separate the modes of solid phase exothermic reactions were defined on the parametric diagram: Todes parameter - Semenov parameter. The results of analytical calculations and numerical analysis are in satisfactory agreement. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.