The reduced Schiff bases of salicylaldehyde [and o-vanillin (o-van)] with D,L- and L-diaminopropionic acid (DPA), designated by salDPA, and L-2,4-diaminopentanoic acid [ornithine (Orn)], designated by salOrn, as well as the VIVO2+ and VVO2+ complexes of salDPA were prepared. The compounds were characterised in the solid state and in solution. The structure of H4salDPA+Cl– was determined by X-ray diffraction. Complexation of VIVO2+ and VVO2+ with salDPA and salOrn (only the VIVO system) in aqueous solution was studied by potentiometry, UV/Visible spectroscopy and circular dichroism, as well as by EPR spectroscopy for the VIVO–salDPA system and by 1H- and 51V NMR spectroscopy for the VVO2–salDPA system. Stoichiometries and complex formation constants were determined by potentiometry at 25 °C and I = 0.2 M KCl. Practically only 1:1 complexes were formed in both systems with composition (VO)LH2 and(VO)L in the VIVO–salDPA system, and with composition(VO2)LH and (VO2)L in the VVO2–salDPA system. Spectroscopic data provided information about the most probable binding modes of each stoichiometry. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)