The deuteration of synthetic calcium hydroxyapatite (CaHap) and the effect of fluoridation on the deuteration were investigated by in situ Fourier transform infrared spectroscopy to elucidate the surface structure of CaHap particles. The deuteration of column OH- ions of CaHap was reduced by the fluoridation, indicating that the deuteration does not happen in the columns of which the entrance is closed with F- ions replacing OH-ions. The 2680-cm(-1) band observed on the deuterated CaHap was assigned to the surface OD- ions which react with CO2 molecules. The 2633-cm(-1) band due to the column OD- ions was not influenced by CO2 adsorption. The fluoridated CaHap did not give rise to the surface OD- band, because the surface OH- ions were replaced with F-.