The e4ect of the metal ions in the chlorapatite structure on the topotaxial conversion to the hydroxyapatite structure was investigated, using three di4erent types of chlorapatite single crystals, calcium, strontium, and barium chlorapatite. The topotaxial conversion to the hydroxyapatite structure was conducted under hydrothermal conditions at a range of temperature from 300 to 8003C for intervals between 3 and 96 h in concentrated KOH solutions. The Cl ؊ ion exchange in the calcium and strontium chlorapatite crystals was completed at 500 and 7003C in a 6.25 M KOH solution, respectively, which resulted in the formation of hydroxyapatite structure. The transformed crystals retained their original shape and dimensions due to the topotaxial transformation. In contrast, the rate of the topotaxial conversion was further reduced on the barium chlorapatite crystals that contain metal ions with large size such as Ba. These crystals were only partially converted to hydroxyapatite even at 8003C in 10 M KOH solution. Under these conditions, a dissolution of ion species of the topotaxially converted barium hydroxyapatite layer was observed, which resulted in the precipitation of new crystals on the surface of the remaining barium chlorapatite crystals. The structure of the new crystals was matched with the hexagonal apatite structure, but a great amount of K was incorporated in the structure.