Although TGA has been employed frequently to analyze reactions of solids that generate volatile products,more information can be obtained by comparing DTG data fromcrucibles of different shape and size. A fast reversible reaction, such as the decomposition of CaCO3, shows an increase in DTG peak temperature with an increase either in the sample height or in the free space volume of the crucible. Consecutive reactions that contain reactive volatile intermediates may show the same DTG temperature with changes in crucible shape and size if the reaction of the volatile intermediates forms no solid or the reactivity of the volatile intermediates for solid is low. Pyrolysis of BLT and SMcoals in TGA using a flat crucible under sufficient purging eliminates the solid formation from the volatiles in the crucible and yields data that may be attributable to the cleavage of covalent bonds that generates volatiles.