Thermal transformations of natural calcium oxalate monohydrate known in mineralogy as whewellite have been undertaken using a combination of thermal analysis and Raman microscopy with the use of a thermal stage. High resolution thermogravimetry shows that three mass loss steps occur at 162, 479 and 684degreesC.Evolved gas mass spectrometry shows that water is evolved in the first step and carbon dioxide in the second and third mass loss steps. The changes in the molecular structure of whewellite can be followed by the use of the in situ Raman spectroscopy of whewellite at the elevated temperatures. The whewellite is stable up to around 161 degreesC, above which temperature the anhydrous calcium oxalate is formed. At 479degreesC, the oxalate transforms to calcium carbonate with loss of carbon dioxide. Above 684degreesC, calcium oxide is formed. (C) 2003 Elsevier B.V. All rights reserved.