We describe a 1D and 2D electron spin echo envelope modulation investigation of VO super(2+) adsorbed on hydroxyapatite (HA) at different concentrations and compare with VO super(2+)-triphosphate (TPH) complexes studied previously in detail, in an effort to provide more insight into the structure of VO super(2+) coordination in bone. Structures of this interaction are important because of the role of bone in the long-term storage of administered vanadium, and the likely role of bone in the steady-state release of vanadium leading to the chronic insulin-enhancing anti-diabetic effects of vanadyl complexes. Three similar sets of cross-peaks from phosphorus nuclei observed in the super(31)P hyperfine sublevel correlation (HYSCORE) spectra of VO super(2+)-HA, VO super(2+)-TPH and VO super(2+)-bone suggest a common tridentate binding motif for triphosphate moieties to the vanadyl ion. The similarities between the systems present the possibility that in vivo vanadyl coordination in bone is relatively uniform. Experiments with HA samples containing different amounts of adsorbed VO super(2+) demonstrate additional peculiarities of the ion-adsorbent interaction which can be expected in vivo. The HYSCORE spectra of HA samples show varying relative intensities of super(31)P lines from phosphate ligands and super(1)H lines, especially lines from protons of coordinated water molecules. This result suggests that the number of equatorial phosphate ligands in HA could be different depending on the water content of the sample and the VO super(2+) concentration; complexes of different structures probably contribute to the spectra of VO super(2+)-HA. Similar behaviour can also be expected in vivo during VO super(2+) accumulation in bones.