Plasma-sprayed hydroxylapatite coatings, widely used on metallic surgical implants to improve their adhesion to bone, are formed by rapid quenching of molten, or partly molten, particles which impact the substrate at high velocity. The performance of these coatings in the body depends upon their structure, which is not well understood. Coatings prepared under a range of spraying conditions have been studied by X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA) and solid-state nuclear magnetic resonance spectroscopy (NMR). The results suggest that particles partly melt and lose combined water at lower plasma torch input powers forming a glass, by quenching of the liquid phase, and an OH-depleted hydroxylapatite residual crystalline phase. At higher power inputs an increasing amount of P205 is also lost and the coatings contain CaO and Ca4P2Og. Heat treatment of coatings in air at 600 ยฐC results in crystallization of the glass phase and reaction with water vapour to form hydroxylapatite. The results show that รRD is relatively insensitive to some of the structural details of hydroxylapatite coatings which may be significant to their performance. NMR provides more structural information and is a significant tool for coating characterization.