Radio frequency (rf) magnetron sputtering of calcium phosphate (CaP) is a promising technique to apply thin bioactive films on bulk implant materials. In this paper the interaction is studied of thin (similar to 100 nm) CaP films with Ca/P ratios ranging from 0.6 to 4.7, with simulated body fluids (SBFs) of different Ca and PO4 concentrations. Analysis of the thickness and composition of the coatings was done with ion beam analysis. Amorphous coatings of all compositions were found to dissolve in all meta-stable SBFs. For coatings with a Ca/P ratio of 1.6, dissolution was linear and congruent. By heat-treating the coatings for 30 min at 650 degrees C, coatings were crystallized. Crystallized coatings with a Ca/P ratio of 1.6 remain inert when immersed in SBF for days. When immersed in doubly concentrated SBF, after an induction period of 40-60 min where virtually no apposition of material is observed, a CaP precipitate is formed. After formation of these CaP crystals, continuous growth can proceed in SBFs with lower Ca and PO4 concentrations. During the induction period, CaP sediments were found on the coating surface. Their role remains unclear, but their presence seems to be correlated with the ability to grow a precipitate layer from the solution. Completion of the induction period is essential for growth of the precipitate. (c) 2004 Elsevier B.V. All rights reserved.