The electrochemically reduced monosubstituted diimine complexes [Ru(bpy)(CN)(4)](2-), [Ru(bpm)(CN)(4)](2-), and [Ru(bpz)(CN)(4)](2-) have been characterized by electron spin resonance spectroscopy. Well-resolved hyperfine structure (hfs) was observed in all three complexes, and coupling constants were calculated from the observed spectra. The hyperfine coupling constants derived from the ESR spectra indicate that the electronic spin density is largely localized within the pi* orbitals of the reduced species. Emission spectra, solvatochromic absorption spectra, and cyclic voltammetry data are also presented and discussed.