It is pointed out that many of the thermal analysis techniques commonly utilized to measure the rates of thermal decomposition reactions are plagued with problems of mass and energy transport limitations. Because the technique employed here, dynamic X-ray diffraction (DXRD), allows for in situ observations of the solid reactants, intermediates, and products, and for better control of the heat and mass transfer resistance, most of these problems are obviated. Results are presented which not only compare the dramatic differences in global reaction rates between TGA and DXRD experiments, but also demonstrate the effects of sample preparation on calcite decomposition. Specifically, it is shown that sample cleaning, which tends to remove nucleation sites, as well as sample de-gassing, which probably removes water vapor, can lead to very different calcite decomposition rates.