The paper gives results of an investigation of the kinetics of the reaction between CaO and HCl under conditions that minimize bulk mass transfer and pore diffusion limitations. Reactivity data from 0.2 to 1 s exposure to 5000 ppm HCl in a fixed-bed reactor were analyzed by a shrinking core model of diffusion and chemical reaction control, either singly or in combination. Between 150 and 350 C, the reaction is controlled by gaseous diffusion through the developing product layer. The apparent activation energy is about 28.1 kJ/mol (6.7 kcal/mol), and the reaction is first order with respect to HCl concentration. Reactivity is a minor function of the measured particle size and surface area, likely due to the agglomerative nature of the individual grains that comprise the particle structure and complicate the interpretation of these measured values. Extrapolation of these results to the high temperature, furnace sorbent injection process provides preliminary agreement with pilot scale tests. (Copyright (c) 1992 American Chemical Society.)