Modification of the chemical composition provides a simple, but powerful approach in tailoring the properties of hydroxyapatite. To date, activity has been directed to measuring the calcium to phosphorous molar ratio, and substitution of calcium or phosphate. Less attention has been directed to the quantity of hydroxyl groups in hydroxyapatite. This is an issue with thermally processed hydroxyapatite, where hydroxyl ion content may be depleted and lead to changes in properties. The experimental section includes: 1. Measurement of the hydroxyl ion content - the preparation of standard hydroxyapatite has been described, the use of thermal gravimetric analysis for quantification of hydroxyl ions has been assessed, calibration curve for the determination of hydroxyl ion amount using Fourier transform infrared spectroscopy has been developed and results compared to Raman spectroscopy. 2. Influence of the hydroxyl ion content on the biological response of hydroxyapatite coatings - hydroxyapatite thermal spray coatings have been prepared and characterized, and the effect of hydroxyl ions on the osteoblast cell response has been studied.