Samples from coral skeletons belonging to six species have been submitted to thermogravimetric analysis in the temperature range between 20¿ and 550¿C. Weight losses are interpreted on the basis of simultaneously recorded infrared absorption spectra at specific wavelengths for water and CO2. The organic + water content of coral skeletons represents up to 2.5% of the total weight. Additionally, it is shown that during heating the main part of water remains linked to organic compounds up to 275¿--330¿C, suggesting organic materials associated with the skeleton are hydrated.