This work investigates surface modification of calcium hydroxyapatite (CaHAP) with organophosphonic acids (RP(O)(OHM)(2)). The solution-phase reactions of RP(O)(OH)(2) (R = n-C8H17, n-C18H37, and n-C8F17(CH2)(2)) were studied with two types of CaHAP substrates: high surface area powder (SN2,BET = 60 m(2)/g) and thin films supported on Ti/Si wafers. It has been found that the concentration of the RP(O)(OH)(2) solution has a dramatic effect on the structure of modified surfaces. Solutions of low concentration (similar to5-10 mM or less) yielded covalently attached monolayers supported on CaHAP. According to Fourier transform infrared spectroscopy, the major products of the reaction were organophosphonic groups bound to the surface via Ps-O-P bonds. Grafting densities of alkyl groups in these monolayers were similar to2.4 groups/nM(2), which is only similar to0.55 of the maximum values typically observed for closely packed self-assembled monolayers, e.g., alkyltrichlorosilanes on silica, indicating a disordered structure of the surfaces. Loose packing of alkyl groups was also supported by adsorption and wettability studies. Solutions of high concentration (similar to10-20 mM or higher) yielded the formation of composites with a high loading of organics believed to be due to the bulk modification of CaHAP. The kinetics of the reactions followed a first-order equation with rate constants ranging from similar to1 1L(mol(.)h) using a good solvent (tetrahydrofuran) to similar to1000L/(mol(.)h) using poor solvents (toluene, ethanol). Modified surfaces showed good hydrolytic and thermal stability. No loss of the grafted material was observed after rigorous washing with solvents and ultrasound, which makes this surface modification technique useful for tailoring the CaHAP surfaces and modifying the adsorption and wettability properties of the CaHAP adsorbents, fillers, and biomaterials.