Using a combination of potentiometry (glass electrode) and quantitative 51V NMR spectroscopy, the full speciation in the vanadate-peroxide and vanadate-peroxide-imidazole systems was determined in the pH range 1-10 (0.150 M Na(Cl) medium, 25 degrees C). Using the computer program LAKE, the pKa value of imidazole and the formation constants for 10 peroxovanadate species and also for three more species where a single imidazole moiety is also bound, have been calculated. The experimental data show a good fit to the calculated speciation model, even for the less abundant species. The species are either monomeric or dimeric in vanadium, and four resonances of the dimeric species have been unambiguously assigned via 2D 51V NMR. Diperoxovanadates are the favoured species at pH 2-10, when sufficient peroxide is present. Imidazole is found to bind strongly to them at pH 6-9. The equilibrium conditions are illustrated in distribution diagrams.