The kinetics of the reactions between mixed powders of BaCO3 and CuO, as well as BaCO3 and Y2O3, have been studied using DXRD techniques as a function of particle size, temperature, and CO2 pressure. Except for initial nucleation phenomena, the reaction rates are governed by shrinking core behavior for BaCO3 particle sizes between 6 and 33 mu m. During the initial stages of the reactions, the surface reaction kinetics are governing, whereas the diffusion of CuO, Y2O3, and CO2 are limiting factors at later stages in the reactions. Quantitative conversion data were used to determine the values of the activation energies and the pertinent diffusivities in these systems.