The water content of black pigment and cholesterol gallstones was evaluated with the use of moisture evolution analysis, electron spectroscopy for surface analysis, and X-ray diffraction. X-ray diffraction identified complex hydrated hydroxyapatite compounds in two stones. Moisture evolution analysis demonstrated that 18 pigment gallstones contained between 0.83 and 6.87% water; six cholesterol stones contained 0 to 0.27% (p < 0.001). When black stones were subdivided into subgroups containing large proportions of pigment, phosphate, or carbonate, the proportion of water evolved at each temperature was the same. Water content was inversely related to carbonate content (r = −0.81) and did not correlate well with other stone components. Electron spectroscopy for surface analysis independently suggested that water of hydration was present since the ratio of oxygen to other elements exceeded ratios in any known compound in stone. A peripheral-central gradient for water was absent in one stone which was analyzed in peripheral mid-, and central portions. Progressive removal of the surface of previously ground stone particles using an argon beam followed by repeated electron spectroscopy for surface analysis indicated that much of the water was present in the outer 20 to 50 Å of the particles. The data are consistent with the hypothesis that pigment stones form by deposition of calcium salts on an organic matrix and that hydrated water is associated with thin layers of calcium salts. These layers may be distributed throughout the stone; powdering appears to fractionate stones through these areas of hydrated calcium salts since oxygen was in highest concentration on the surface of powder particles.