Millimeter-sized hydroxyapatite (HA) single crystals were synthesized from chlorapatite (ClAp) crystals via the ionic exchange of Cl for OH at high temperature. X-ray diffraction, Fourier-transform infrared spectroscopy, and chloride content measurements were used to follow the progress of this conversion, and to assess the effect of the experimental conditions (temperature, time, and atmosphere). ClOH exchange took place homogeneously and was enhanced by firing in wet air. After firing at 1425 degrees C for 2h 92% of the Cl ions were exchanged by OH while maintaining crystal integrity. Temperatures above 1450 degrees C damaged the surface of the crystals, destroying the hexagonal habit at 1500 degrees C. The composition of these apatite crystals was close to bone mineral content. Their nanoindentation hardness (8.7 +/- 1.0GPa) and elastic nanoindentation modulus (120 +/- 10GPa) were similar to those of the starting ClAp (6.6 +/- 1.5GPa, and 110 +/- 15GPa, respectively). However, their average flexural strength was similar to 25% lower due to the formation of defects during the thermal treatments.