The formation of complexes of Co^II with violuric acid (1H,3H-pyrimidine-2,4,5,6-tetrone 5-oxime), H₃vi, and its monomethyl (H₂mvi) and dimethyl (Hdmvi) derivatives in dimethyl sulphoxide–water (80 : 20) has been investigated. Potentiometric, calorimetric, magnetic, and e.s.r. measurements clearly evidence that [Co(H₂vi)₃]^–, [Co(Hmvi)₃]^–, and [Co(dmvi)₃]^– are low-spin octahedral complexes. The high spin–low spin change occurs upon binding of the third ligand.