The dinuclear organoplatinum compounds RnPt(µ-BL)PtRn (R = Me = methyl, n = 2,4; R = Mes = mesityl, n = 2; µ-BL = the bridging ligands 2,2′-bipyrimidine (bpym), 2,5-bis(1-phenyliminoethyl)pyrazine (bpip) or 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) undergo reversible one-electron reductions to yield the corresponding radical anions [RnPt(µ-BL)PtRn]˙− The chemically or electrochemically reduced species were studied by continuous wave EPR spectroscopy at various microwave frequencies (4–34 GHz). With one exception, only a single broad line is observed in fluid solution at ambient temperatures and X-band. Measurements on glassy frozen solutions at 110 K at S-, X-, K- or Q-band reveal not only all the three g-values for the rhombic spectra but also most of the hyperfine coupling constants to the 195Pt nuclei and also to two 1H nuclei of the bridging ligand when µ-BL = bpym. Density functional–Hartree–Fock (DFT–HF) molecular orbital calculations show the singly occupied molecular orbital in each case to be predominantly bridging-ligand based, in good agreement with the magnitudes of the observed 195Pt hyperfine coupling constants. The molecular structure of the free ligand 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) was determined by X-ray crystallography to serve as a basis for the calculations.