The surface of synthetic calcium hydroxyapatite Ca-10(PO4)(6) (OH)(2) (CaHAP) particles was modified with hexanoic CH3(CH2)(4) COOH (HA) and decanoic CH3(CH2)(8)COOH (DA) acids in hexane and the resulting materials were characterized by various means. XRD pattern, specific surface area, and morphology of the CaHAP particles were essentially not altered by the modification, FTIR and TPD results indicated that HA and DA molecules are hydrogen-bonded to the surface P-OH groups of CaHAP in three adsorption types. The surface of modified particles became hydrophobic. When the modified samples were outgassed at 500 degrees C, HA and DA were removed and the surface P-OH groups were revived. The modified material adsorbed much less H2O and CO2 than the unmodified one.