Ionic nickel(IV)-dimethylglyoxime complexes have been prepared by oxidation of nickel(II)-dimethylglyoxime in alkaline solutions with ferricyanide. A novel base-controlled equilibrium is demonstrated between the monomeric and dimeric nickel(IV)-dimethylglyoxime complexes in solution. The oxidation state of nickel as +4 is confirmed by titration with hydroxylamine salt. The results obtained from the UV/VIS spectrophotometry, titrimetry, infrared and NMR studies are discussed. © 2008 Springer Science+Business Media B.V.