High-temperature chemical reactions in mixtures of solid CaSO4 and Na2CO3 were investigated in order to explore the mechanisms of enhanced sulfur capture by limestones doped with Na2CO3 in fluidized bed combustion. Drastic weight loss of the mixtures was observed in a thermogravimetric analyzer near the melting temperature of Na2CO3, indicating chemical reaction. X-ray diffraction analysis for a mixture of the solids following a heat treatment at 850 degrees C revealed the existence of two sodium carbonate sulfate double salts that have not been reported before for the present system. The formation of Na2SO4 in the melt of Na2CO3 appears to precede the formation of the double salts. The two double salts are believed to have high porosity and specific surface area similar to those of a better-known double salt, burkeite. The implications of these findings for the enhancement of limestone sulfation by Na2CO3 are also discussed. (c) 2007 Published by Elsevier B.V.