The speciation in the aqueous H(+)/H(2)VO(4)(-)/phosphate (dihydrogen phosphate, P(-)) and H(+)/H(2)VO(4)(-)/H(2)O(2)/P(-) systems has been determined in the physiological medium of 0.150 M Na(Cl) at 25 degrees C. A combination of multinuclear NMR integral and chemical shift (Bruker AMX500) as well as potentiometric data (glass electrode) have been collected and treated simultaneously by the computer program LAKE. The pK(a)-values for phosphoric acid have been determined by potentiometric and (31)P NMR chemical shift data, and have been found to be 1.85 +/- 0.02, 6.69 +/- 0.02 and 11.58 +/- 0.07. The errors given are 3sigma. Altogether nine vanadate-phosphate species have been found in the ternary H(+)/H(2)VO(4)(-)/P(-) system in the pH region 1-11, with the following compositions: VP, VP(2) and V(14)P. Equilibrium is very slow in acidic solutions, requiring more than 3 months for the formation of V(14)P species. On the other hand, less than 15 min are needed for equilibration at neutral and alkaline pH. In the quaternary H(+)/H(2)VO(4)(-)/H(2)O(2)/P(-) system, four new species have been found in addition to all binary and ternary complexes. They are of VXP and VX(2)P compositions, where X denotes the peroxo ligand. (51)V and (31)P NMR chemical shifts, compositions and formation constants are given, and equilibrium conditions are illustrated in distribution diagrams as well as the fit of the model to the experimental data. Biological and medical relevance of the species is also discussed and physiological conditions are modelled.