The speciation in the quaternary aqueous H + -H 2 VO 4 - -H 2 O 2 -l-α-alanyl-l-histidine (Ah) system has been determined from quantitative 5 1 V NMR measurements and potentiometric data (glass electrode). The study was performed in 0.150 M Na(Cl) medium at 25 o C. Data were evaluated with the computer program LAKE, which is able to treat combined potentiometric and NMR data. In the ternary H + -H 2 VO 4 - -Ah system, two complexes, (H + ) p (H 2 VO 4 - ) q (Ah) r , having (p, q, r) values (0, 1, 1) and (1, 1, 1) (pK a =6.88) explain all data. In the quaternary H + -H 2 VO 4 - -H 2 O 2 -Ah system, seven complexes were determined in addition to all binary and ternary complexes, four with a V/X/Ah ratio 1:1:1 and three with a ratio 1:2:1 (X=peroxo ligand). VX 2 Ah 2 - and VX 2 Ah - (pK a =8.26) are the main quaternary complexes and predominate in the pH range 5 to 9. Chemical shifts, compositions and formation constants for all the quaternary complexes are given, and equilibrium conditions are illustrated in distribution diagrams.