The pyrimidinones mhcpe, 2-methyl-3H-5-hydroxy-6-carboxy-4-pyrimidinone ethyl ester (mhcpe, 1), 2,3-dimethyl-5-benzyloxy-6-carboxy-4-pyrimidinone ethyl ester (dbcpe, 2) and N-methyl-2,3-dimethyl-5-hydroxy-6-carboxyamido-4-pyrimidinone (N-MeHOPY, 3), are synthesized and their structures determined by single crystal X-ray diffraction. The acid-base properties of 1 are studied by potentiometric and spectrophotometric methods, the pK a values being 1.14 and 6.35. DFT calculations were carried out to determine the most stable structure for each of the H 2 L + , HL and L − forms (HL = mhcpe) and assign the groups involved in the protonation-deprotonation processes. The mhcpe − ligand forms stable complexes with V IV O 2+ in the pH range 2 to 10, and potentiometry, EPR and UV-Vis techniques are used to identify and characterize the V IV O-mhcpe species formed. The results are consistent with the formation of V IV O,