Both monomeric and dimeric forms of the oxoosmium(VI) complexes formed by reaction between oxoosmium(VI) species and N, N', N'-tetramethylethylenediamine have been isolated and characterized by elemental analysis, by IR, UV, and NMR spectroscopy, and by molecular weight determinations. Equilibrium constants have been measured as a function of temperature. The rate of the dimer to monomer conversion has been measured as a function of pH. Corresponding 1, 10-phenanthroline complexes have also been isolated and partially characterized largely through study of their IR spectra since, in this case, the equilibrium and solubility characteristics make studies in solution difficult. © 1979, American Chemical Society. All rights reserved.