Highly porous microcrystalline hydroxyapatite (HAP) coatings have been prepared from calcium nitrate and phosphoric acid based sols by the aerosol-gel process. The coatings were studied after sintering at different temperatures with the use of Fourier transform infrared spectroscopy, x-ray diffraction, energy disperse x-ray microanalysis, scanning electron microscopy, and transmission electron microscopy. The composition, structure, and morphology of the coatings sintered at 650 °C fit fairly well highly porous HAP. These coatings were reproduced onto TiO2/Si substrates and studied by Rutherford backscattering. It is shown that even after chemical etching, an adherent calcium phosphate phase remains attached to the TiO2/Si substrate.