A systematic study of the physiologically interesting vanadium-maltol (V-MaH) system has been performed in 0.150 M Na(CI) at 25 degrees C, using NMR, ESR, and potentiometric techniques. Complexation occurs within a wide pH range, from around 1 up to 10.5. However, a pH-, concentration-, and time-dependent spontaneous reduction of vanadium(V) to vanadium(IV) occurs. From ESR spectra the conditions for this reduction are evaluated and discussed. From potentiometric (glass electrode) and quantitative V-51 NMR measurements, the full speciation in the H+-H2VO4-MaH system was determined in the pH range 5-10.5. Data were evaluated with the computer program LAKE, which is able to treat combined emf and NMR data. The pK(a) value for MaH was determined to be 8.437 +/- 0.005. In the ternary system, three complexes are formed : VMa(2)(-), VMa(-), and VMa(2-), having log beta(0,1,2) = 7.02 +/- 0.03, log beta(0,1,1) = 2.66 +/- 0.05, and log beta(1,1,1) = -7.37 +/- 0.21. The errors given are 3 sigma. The VMa(2)(-) complex appears as the main species in a pH range from 4.5 to 8.5, whereas both mononuclear monoligand species are minor. Equilibrium conditions are illustrated in distribution diagrams, and the structures of the complexes formed are proposed.