This work describes a mathematical model for a gas-solid reaction taking place within reactant particles made up of very small spherical grains, which grow during the reaction process due to solid product formation around the grain inner core, so that the particle void-fraction is reduced as the reaction proceeds. The structural changes in the spherical grains are taken into account in the proposed model by the inclusion of a variable diffusion coefficient of the gaseous reactant through the developing product layer. This progressively increasing transport resistance leads to a marked reduction in both reaction rate and sorbent capacity, as observed in practice in the carbonation of calcined dolomite, a process of importance in hydrogen production by means of coal or biomass gasification. (C) 2009 Elsevier Ltd. All rights reserved.