Understanding the interaction of bioactive coatings with aqueous media is essential for development of systems possessing rapid osteointegration and durability. An in vitro study of a commercial, plasma sprayed hydroxyapatite coating has been undertaken. The coating behaviour when test coupons were immersed in water, simulated body fluid and foetal calf serum has been examined. The principal aim was to characterise, in detail, any structural changes to the coatings and, in particular, examine features of any new layers formed. The amorphous phase of the coating showed preferential dissolution in all media. The rate of dissolution was greatest in water and the process was initially retarded in the foetal calf serum. A nanocrystallite apatite layer was shown to precipitate on the coatings in all media although this was significantly enhanced in simulated body fluid. The features of this layer (e.g., lattice parameters, crystallite size etc.) were quantified by adopting a novel approach to the X-ray diffraction data analysis. The results are discussed in the context of similar studies and implications for in vivo behaviour. (c) 2005 Springer Science + Business Media, Inc.