In order to improve the gelation in sol-gel preparation of hydroxyapatite by using Ca(NO3)(2) and PO(OH)(3-x)(OEt)(x) precursors, citric acid was selected as an enhancing gelation additive. Hydroxyapatite derived from the mixed precursor solutions (Ca/P molar ratio of 1.67) with citric acid showed a different reaction path from that without citric acid. The addition of citric acid led to formation of amorphous as-prepared powder, the powders calcined at 650 degreesC contained three phases: hydroxyapatite, carbonated hydroxyapatite and a small amount of P-tricalcium phosphate. The powders calcined at 800 degreesC contained only a single phase of oxy-hydroxyapatite. It is suggested that citric acid plays a role in enhancing gelation through a strong coordination ability of Ca ions with citrate groups. (C) 2002 Elsevier Science B.V. All rights reserved.