The thermal decompositions of sodium hydrogen carbonate, calcium oxalate monohydrate, and the compounds NiL4Cl2(L = pyridine, 3- and 4-methylpyridine) have been studied with a differential scanning calorimeter. Activation energies, orders of reactions, and pre-exponential factors have been calculated from the calorimetric data by a least-squares computer program. The results for the decompositions of sodium hydrogen carbonate and calcium oxalate monohydrate are compared with those obtained by other workers. The rate-determining step in the decompositions of the nickel complexes is shown to be metal–ligand bond cleavage.