Studies of a wide variety of modern and fossil skeletal carbonates indicate they all contain water in amounts ranging from a few tenths of one percent to as much as three percent by weight. The word water is used as a general term to include all inorganic liquid and bound H 2 O and OH (super -) . Reflectance spectroscopy in the visible and near infrared (0.60 to 2.55 mu m) reveals major differences among skeletal carbonates in the amount of water present, phases in which it occurs, and relative proportions of these phases. Liquid H 2 O is present in all skeletal material studied in the form of fluid inclusions. In some cases, most notably the molluscs, H 2 O is also associated with an organic matrix. A hydrated CaCO 3 phase occurs in some biogenic low-Mg calcites (LMCs) and aragonites, and two hydrated MgCO 3 phases with differing bond energies occur in high-Mg calcites (HMCs). Many HMCs contain Mg(OH) 2 and H 2 O bound to CaCO 3 as well, and some biogenic aragonites contain Ca(OH) 2 . Water content varies with skeletal mineralogy, taxonomic group, and, in the case of the molluscs, with skeletal structure. Alteration of aragonites and HMCs to LMCs is accompanied by order-of-magnitude changes in total water content. HMCs lose bound H 2 O and OH during alteration to LMCs. Ca(OH) 2 and H 2 O bound to CaCO 3 may also be lost from biogenic aragonites, even in the absence of changes in bulk mineralogy. LMCs also lose H 2 O bound to CaCO 3 during diagenesis. Fluid inclusions are lost during neomorphic alteration of aragonites and LMCs. However, skeletal material that retains its original bulk skeletal mineralogy and/or original microstructure as seen in thin section also retains the bulk of its original fluid inclusion content. Water content can be used as a criterion for monitoring diagenetic alteration in carbonates. Water present in fluid inclusions can serve as a medium for diagenetic reactions. Bound H 2 O lost on heating could take part in diagenetic reactions during deep burial as well.