The effects of external factors on the overall rate of the gas-solid reactions observed in a thermogravimetric analyzer (TGA) were determined for the reaction involving calcined limestone (i.e., CaO) and hydrogen sulfide. The external factors, which are dependent on the experimental setup, included the sample weight, gas flow rate, particle dispersion, and sample bed geometry. The experiments were conducted using three different sample holder geometries, 10 sample weights (1-500 mg), and two reactant gas flow rates (500 and 850 sccm). The results show that these external factors can lead to measurements of the overall rate of gas-solid reactions that can span over 2 orders of magnitude under identical reaction conditions. The effects of the reactant gas concentration profile and effective interparticle diffusion were determined to calculate the solid-dependent overall reaction rate. The measured and calculated overall reaction rates were compared with data reported in the literature for experiments performed using TGA as well as flow-through differential-bed reactors.