In determining char reactivity using thermogravimetric analysis (TGA), typically the sample is heated in an inert atmosphere until the reaction temperature is achieved and the gas is switched to the reaction gas. The reaction gas initially purges the inert gas from the furnace. The main objective of this work is to determine the role of "furnace purging" in determining the rate parameters of porous solid-gas reactions. Various tests were conducted in this study using char samples with different time-temperature-conversion histories and gas environments in TGA. Increasing the air flow rate to purge the inert gas resulted in achieving the maximum rate much earlier. Results showed that the observed maximum during the rate measurement is the result of a furnace purging effect, and a mathematical expression was developed to correct for the time it takes to purge the inert gas to build the partial pressure depending on the volume of the furnace. This correction, when applied to the reactivity profile, resulted in an increase in the activation energy of about 3-25 kJ/mol, depending on the extent of conversion. This result suggests that estimating rate parameters or deducing conclusions regarding char reactivity from the initial part of a rate profile is misleading if the purging effect is ignored.