The effects of steam (H2O) and carbon dioxide (CO2) on the rates of calcite decomposition have been studied using dynamic X-ray diffraction (DXRD). Because the DXRD technique allows for considerably better control of heat and mass transfer complications, intrinsic decomposition rates are obtained for temperatures between 440-560 degrees C and up to H2O and CO2 pressures of 0.2135 and 0.00087 atm, respectively. It is shown that the observed enhancement effect of steam is related to its adsorptive properties which are faster and more significant than CO2 adsorption. This, in turn, leads to the hypothesis of a quantitative Langmuir-Hinshelwood model which is totally compatible with the kinetic data. Separate adsorption experiments show that the effect of steam pressure is limited due to adsorption saturation capacity and the model is used to predict the steam pressure required to produce 95% of the maximum effect of steam as a function of temperature.