RF magnetron sputtering of calcium phosphate (CaP) coatings is a promising technique to apply thin bioactive films on bulk implant materials. In this paper the properties of the interface between RF sputtered coatings and simulated body fluids (SBFs) are related to the ability to form Cap crystals on the coating surface. Two types of coatings were compared: coatings with a low Ca over P ratio (similar to 0.8; CaPlow), which remain inert when immersed in SBF, (i.e. SBF with twice the Ca and PO4 concentrations), and coatings with a high Ca over P ratio (1.6; CaPhigh), which show the formation of CaP crystals on their surface within 2 h. Low energy ion scattering (LEIS) and radioactive labeling of the SBFs combined with liquid scintillation counting (LSC) allowed us to study very accurately the composition of the adsorbates of both coating groups after 10 min of immersion in SBF2. For the adsorbate on CaPhigh and CaPlow coatings coverages were found consistent with ionic adsorption and CVP ratios of 1.24 +/- 0.02 and 2.17 +/- 0.10, respectively. Adsorption was found to be reversible over the studied immersion period. After an induction period of 40 min a CaP precipitate started to form on the CaPhigh coatings with a Ca/P ratio of 1.30 +/- 0.02. Further, no significant desorption of coating species was observed during this induction period. (c) 2005 Elsevier B.V. All rights reserved.