A theoretical analysis of thermal processes in liquid-gas exothermic reactions in a porous catalyst shows the existence of the alternating motion of liquid in the pore. According to this model, if the released heat in the pore exceeds a certain critical value, the alternating motion of the liquid driven by the formation of the bubble takes place in the pore. Specifically, because of the heating of the liquid in the pore, the partial pressure of vapor saturated in liquid increases until the total pressure of the saturated gas and vapor becomes greater than the maximum possible pressure in the pore (equal to the sum of capillary pressure and pressure in the reactor) and the bubble thereby comes into being. The growing bubble pushes the liquid out of the pore. Since the reagent(s) is no longer in the pore, the reaction ceases and the generated heat dissipates. The liquid penetrates into the pore due to capillary force and the process of bubble formation occurs once again. A detailed analysis of this non-stability is undertaken. This paper seeks to show how this theory can explain some dependencies of the reaction rate observed in practice, mass and heat transfer in the fixed and suspended catalyst as well as some practical recommendations for catalyst development and process intensification. Analogy between oscillation behaviour and the boiling process is considered. (C) 2003 Elsevier Science B.V. All rights reserved.