Inorganic carbonates, which occur abundantly on earth, constitute an inexpensive natural source of carbon. Therefore, the direct conversion of these carbonates into methane is of considerable importance. Thermal decomposition of transition metal carbonates with the composition MCa(CO3)2 (where M=Co, Ni, or Fe, and M/Ca is 1:1) and M1M2Ca(CO3)3 (where M1M2=CoNi, NiFe, or FeCo, and M1/M2/Ca is 1:1:2) shows that the reduced transition metals in combination with metal oxide nanoparticles (e.g., Co/CoO/CaO) act as catalysts for the conversion of CO2 (produced from the carbonates) into methane. The favorable decomposition conditions include heating at 550 °C in an H2 atmosphere for 5–6 h. These catalysts are found to be excellent for the methanation of CaCO3, exhibiting high efficiency in the utilization of H2 with 100 % conversion and 100 % selectivity. The best catalyst for conversion of CaCO3 into CH4 is Co/CoO/CaO. There are also indications that similar catalysts based on Fe may yield higher hydrocarbons.