The surface area of biomimetically deposited hydroxyapatite (HA) coatings on metallic implants is important for the biological performance of the implant. Thus, a nondestructive method of assessing this quantity directly on the solid substrate would be highly valuable. The objective of this study was to develop such a method and for the first time assess the evolution of surface area of HA during biomimetic growth. The surface area of a TiO2-covered titanium substrate was measured prior to and following the biomimetic coating deposition using Ar gas adsorption at 77 K. The presence of HA on the surface was verified with scanning electron microscopy and X-ray diffraction. The specific surface area of the coating was found to increase linearly during 1 week of deposition at a rate of similar to 100 cm(2) day(-1) (g substrate)(-1). The presented method may be used as a toot for studying the evolution in surface area of coatings on solid substrates during biomimetic deposition or other growth processes.