The surfaces of single crystals and polycrystalline specimens of hydroxyapatite were measured by auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Compared to P content, Ca content was found to be enriched near the surface of the hydroxyapatite single crystal. A Ca-deficient and carbonate-containing phase was observed at the grain boundaries of the polycrystalline specimen. Ca content at fracture surface increased as a function of time. It is suggested that Ca ions in the hydroxyapatite crystals migrated from inside the crystal to the surface, and that carbonate ions were incorporated from the gas phase into the crystal interior.