Complexes [Os(pap)(2)(Q)] (1-4) have been obtained and structurally characterized for pap = 2-phenylazopyridine and Q = 4,6-di-tert-butyl-N-aryl-o-iminobenzoquinone (aryl = phenyl (1), 3,5-dichlorophenyl (2), 3,5-dimethoxyphenyl (3), or 3,5-di-tert-butylphenyl (4)). The oxidized form (3)(ClO4)(2) was also crystallographically characterized while the odd-electron intermediates [Os(pap)(2)(Q)](+) (1(+)-4+) and [Os(pap)(2)(Q)](-) (2(-)) were investigated by electron paramagnetic resonance (EPR) and UV-vis-NIR spectroelectrochemistry in conjunction with density functional theory (DFT) spin density and time-dependent DFT (TD-DFT) calculations. The results from the structural, spectroscopic, and electrochemical experiments and from the computational studies allow for the assignments [Os-II(pap(0))(2)(Q(0))](2+), [Os-II(pap(0))(2)(Q(center dot-))](+), [Os-IV(pap(center dot-))(2)(Q(2-))], and [Os-II(pap(center dot-))(pap(0))(Q(2-))](-), with comproportionation constants K-c approximate to 10(3.5), 10(10), 10(18), and 10(5), respectively. The redox potentials and the comproportionation constants exhibit similarities and differences between Ru and Os analogues. While the Q:based redox reactions show identical potentials, the more metal-involving processes exhibit cathodic shifts for the osmium systems, leading to distinctly different comproportionation constants for some intermediates, especially to a stabilization of the neutral osmium compounds described in this article. The example [Os(pap)(2)(Q)]" illustrates especially the power of combined structural and EPR analysis with support from DFT towards the valence state description of transition metal complexes incorporating redox non-innocent ligands.