The autocausticizing reactions of sodium carbonate/borate mixtures have been studied by simultaneous TG/DTA. The main reaction occurs in a molten state of the salts, where the carbonate and borate react to form CO2 and more sodium-rich borates. The extent of decarbonation of Na2CO3 is dependent on the amount of added boron, the temperature, and the composition of the surrounding gas atmosphere. At an overall molar ratio of B/Na <1 and at temperatures of 900-1000 degreesC, the borate composition of the melt is intermediate to NaBO2 and Na3BO3. The decarbonation reaction is favored by high temperatures and low CO2 contents in the gas phase. The composition of the borate components in the melt shifts toward the Na3BO3 composition at high temperatures, low CO2 partial pressures, and low boron additions. If the surrounding gas contains CO2, the salt melt will be recarbonated as the temperature is lowered and the melt crystallizes.