Ion exchange reactions of brown and sub-bituminous coals with natural soda ash, which is composed of > 99 % Na2CO3, have been studied at 20 - 40 °C without any pH-adjusting reagents, and the pyrolysis and subsequent steam gasification of the resulting Na-exchanged coals has been carried out with a fixed bed quartz reactor mainly at 700 °C. When the Na+ concentration and pH of an aqueous mixture of each coal and the soda ash are monitored in the ion exchange process, these factors decrease both at a larger rate with the brown coal that contains a higher content of COOH groups, showing that the ion exchange of Na+ with the H+ of the COOH takes place predominantly. About 65 % of the COOH can be exchanged with Na+ ions under the optimized conditions, irrespective of the coal type. The reactivity of these raw coals in steam at 700 °C is almost the same to be low, and char conversions are less than 20 mass % even after 2 h reaction. The exchanged Na promotes remarkably the gasification of both coals at this temperature, but the rate profile is different: The conversion for the brown coal increases linearly with increasing time and reaches almost 100 % at 1 h, whereas it needs approximately 2 h for the sub-bituminous coal to be gasified completely. The temperature dependency of the conversion with this coal reveals that the use of the Na catalyst can lower reaction temperature by about 120 °C, and the Arrhenius plots of the initial specific rate show that apparent activation energies are estimated to be 190 and 120 kJ/mol without and with the catalyst, respectively. The SEM-EPMA and XRD measurements of Na-bearing chars recovered after the pyrolysis and gasification suggest that the Na catalysts are finely dispersed at the initial stage of the reaction but that they may be deactivated by the formation of sodium silicates at high char conversions of more than 90 % even at a low temperature of 700 °C.