In order to develop a method to estimate the effective thermal conductivity of reactor beds for gas-solid chemical heat pumps, experiments on heat transfer and structure in the bed have been performed for the reaction of CaCl2 with CH3OH or CH3NH2 vapor. From the measurements with mercury porosimetry, it has been found that the void fraction inside the porous CaCl2 particles was influenced considerably by the gas substance reacted with the particles. SEM observation revealed that the shape and aggregated state of fine grains of CaCl2 also changed by reaction. Heat transfer experiments of the CaCl2 bed have been carried out at the gas-phase pressure in the packed bed from 5 Pa to 0.1 MPa. The effective thermal conductivity of the bed, which had been subjected to the absorption and desorption reaction several times, was reduced to less than one half of that of the unreacted CaCl2 bed. A model is proposed to predict the effective thermal conductivity of the reactor bed as well as that of the reactive pellets packed in the bed by taking account of their structural change due to reaction. From a comparison with the experimental results it has been proved that the predictions based on the model well express the effects of gas pressure and the structural change in reactive pellets on the effective thermal conductivity.