4,5,6,7-tetrahydro-5,7-dioxo-[1,2,4]triazolo-[1,5-a]pyrimidine (H₂tpO₂) has been synthesized as well as its monosodic salt, the crystal structure of the latter having been determined by X-ray diffraction. H₂tpO₂ possesses a very acidic active methylene group, as indicated by its dissociation constant (pK_a1 = 2.9). One of the hydrogen atoms in this group could then migrate to a basic position of the molecule, generating a zwitterionic or enolic form, nevertheless molecular orbital calculations point to the tautomer that keeps the CH₂ group as the most stable one. The anionic form HtpO₂^- is generated by the elimination of one of the protons at C6 rather than that attached to N4, as indicated by NMR data and also by the X-ray diffraction data of Na(HtpO₂)·2H₂O, this being consistent with theoretical calculations. The role of the molecular orbitals of HtpO; in possible coordination to metal ions is discussed.