Synthetic carbonate-containing and carbonate-free hydroxyapatites(CO3-HA and CO3-free HA) have been investigated using ESR and IR absorption, together with X-ray diffraction and chemical analyses. Comparison of the IR spectra between 13C-enriched 13CO3-HA and 12CO3-HA prepared at 100 degrees C and at high pH suggested that the carbonate substitutes for PO4 but not for HPO4. The substitution of CO3 for PO4 in this case resulted in elimination of hydroxyl ions in a hexad axis. The elimination of hydroxyl ions was found to be related to the decrease in crystallinity for the c-axis direction, although so far there has been no direct evidence for the existence of hydroxyl deficiencies. In the case of CO3-free HAs prepared at 80 degrees, 60 degrees, 40 degrees, and 20 degrees C, the decrease in crystallinity accompanied by the decrease in the temperature for preparation was associated with the decrease in crystal size, and no significant elimination of hydroxyl ions would be expected in the absence of the carbonate.