Nesquehonite (MgCO3.3H(2)O) (N) can be obtained from a dolomitic quicklime paste in a CO2-rich atmosphere. Thermal decomposition of this synthetic nesquehonite has been studied by TG-DTA analysis. It is very similar to the hydromagnesite (HY) thermal decomposition, as it show similarities in the decarbonations at 440 and 550degreesC, and an exothermic phenomenon at 510degreesC. It has been reported in the literature that some intermediate in N --> HY transformation occur, but this process cannot be directly detected during the heating. Samples were heated at 115, 230, 280, 370, 460, 520, 600, 800 and 1000degreesC and XRD and FIF-IR were used in order to determine the structural changes in nesquehonite and the intermediate phases formed. Results show that nesquehonite transforms at lower temperatures (115 degreesC) into a stable amorphous magnesium carbonate with chemical composition very close to that of HY. Thermal decomposition of nesquehonite, during a gradual temperature increase, proceeds via the formation of this compound. At higher temperatures (460degreesC/short heating times), nesquebonite transforms into HY. The occurrence of an exothermic peak at 510degreesC has been also discussed. (C) 2004 Elsevier B.V. All rights reserved.