A theoretical analysis of liquid-gas reactions with heat or gas production on a porous catalyst shows the possibility of pulsating motion of liquid in pores caused by the formation of bubbles inside pores due to gas or vapor generation. The criterion number determining the conditions under which such behavior occurs is given. Since the mass transfer and chemical reaction in pores take place under forced, oscillating motion, the description of such a phenomenon cannot be made by the Thiele model. The approach to modeling for such processes is being developed. (C) 2003 Elsevier Science B.V. All rights reserved.