Abstract Thermogravimetric analysis (TGA) is an analytical technique in which changes in physical and chemical properties of materials are measured as a function of temperature and/or time. TGA is commonly used to determine selected characteristics of materials that exhibit either mass loss, or gain, because of decomposition, oxidation, or loss of volatile material, such as moisture. Common applications of TGA are materials characterization through analysis of characteristic decomposition patterns and determination of combustible materials and combustion residues from a sample. Macro TGA systems that use gram-size samples have been used for a couple of decades for monitoring industrial processes. The larger sample sizes allow more accurate mass measurements for characterization of materials. Flue-gas desulfurization (FGD) solids are industrial byproducts that have been monitored using macro TGA systems. Compounds reported to have been successfully measured include free moisture, moisture in hydrates, CaSO4•2H2O, CaSO3•½H2O, and CaCO3. A LECO TGA701 macro TGA system was used in a study to characterize the components in various FGD solids. Pure materials, including those listed above, were first analyzed with the TGA701 followed by the measurement of various mixtures of the pure materials. Although Ca(OH)2 is a material present in limestone slurries used in flue-gas desulfurization, previous studies have not indicated the presence of this compound in FGD solids. The TGA reaction profiles for Ca(OH)2 and CaSO3•½H2O show they decompose in the same temperature range. Pure CaSO3•½H2O is not available and was not used in the study. The study of pure materials and mixtures demonstrated the accuracy of the TGA701 in characterizing the compounds. A macro TGA analysis program developed during this study can be used to accurately quantify the various materials in flue-gas desulfurization FGD solids. Carbon and sulfur analysis data support the macro TGA results for the FGD solids.