Ferric, cobalt, and nickel salts of mellitic acid have been prepared as crystalline hydrates which, when heated in vacuo, dehydrate to give almost amorphous substances. On heating in vacuo, the cobalt and nickel salts decomposed completely in a single rate process to yield carbon dioxide as the main gaseous product with smaller amounts of carbon monoxide. The reactions obeyed the second-order kinetic equation, with energies of activation 45·6 ± 1·0 and 66·0 ± 1·0 kcal./mole for the nickel and the cobalt salts, respectively. The solid residue consisted of finely divided metal. The ferric salt decomposed differently in two temperature ranges. The low-temperature reaction, 220–300°, gave carbon dioxide by a second-order reaction having energy of activation 43·5 ± 1·0 kcal./mole, the yield of gaseous products was close to ⅓ that give by the completed reaction. The high-temperature reaction, 425–550°, was zero-order over a substantial fraction of the reaction and the energy of activation was 41·0 ± 1·0 kcal./mole. Iron carbide was the final solid product. Mechanisms of reaction to account for these observations are proposed.