In pure hydroxyapatite the O–H stretching and librational modes give rise to i.r. bands at 3 573 and 631 cm–1. On introduction of F, the band at 631 cm–1 shifts to higher wavenumbers, ultimately to 641 and 647 cm–1, and decreases in intensity. This shift can be correlated with the number n of OH groups in the chain sections between F which have the generalised form ⋯ F HO (HO)n HO:OH (OH)n OH F ⋯. The ‘tail-to-tail’ configuration, ⋯ HO:OH ⋯, gives rise to bands at 680 (bending) and 3 643 cm–1(stretching). The OH bonded to one F gives rise to bands at 720 and 3 546 cm–1. If OH is dispersed in a chain composed mainly of F, the configuration ⋯ F OH F ⋯, the O–H stretching and bending bands lie at 3 540 and 747 cm–1 respectively.