The speciation in the quaternary aqueous H /H 2 VO 4 À /H 2 O 2 /l-aalanyl-l-histidine (Ah) system has been determined from quantitative 51 V NMR measurements and potentiometric data (glass electrode). The study was performed in 0.150 m Na(Cl) medium at 25 8C. Data were evaluated with the computer program LAKE, which is able to treat combined EMF and NMR data. The pK a values for Ah were determined as 8.06, 6.72 and 2.64. In the ternary H / H 2 VO 4 À /Ah system, two complexes, (H ) p (H 2 VO 4 À ) q (Ah) r , for which (p, q, r) values of (0, 1, 1) and (1, 1, 1) with log b 0,1,1 2.52 AE 0.03 and log b 1,1,1 9.40 AE 0.05 (pK a 6.88), respectively, explain all data. The errors given are 3s. In the quaternary H /H 2 VO 4 À /H 2 O 2 /Ah system, eight complexes were determined in addition to all binary and ternary complexes, four with a V/X/Ah ratio 1:1:1 and four with a ratio 1:2:1 (X peroxo ligand). VX 2 Ah 2À and VX 2 Ah À (pK a 8.19) are the main complexes and predominate in the pH range 5 to 9. Three additional minor species have also been found but their compositions could not be determined owing to their small amounts. Equilibria are slow, significant decomposition of peroxide occurs only in acidic solutions. Data in the pH range 5 to 10 have been used for the LAKE calculations. Chemical shifts, compositions, and formation constants for the eight quaternary complexes are given, and equilibrium conditions are illustrated in distribution diagrams. Structural proposals for VX 2 Ah 2À and VX 2 Ah À are made from 1 H and 13 C NMR measurements.