The state and problems of the electrochemical controls of oxidation-reduction systems in aqueous-organic and nonaqueous solvent media with particular emphasis on the rH index of the reducing power of such systems are discussed. In the frame of a generalized treatment of the rH-metric domain, the key definitions, the acquisition of primary rH_S standards, the comparison of rH scales in different solvent media, and the recommended procedure of rH_X determination are described. The most recent results here obtained in the studies of the behaviour of the quinhydrone electrode with respect to the hydrogen gas electrode, and of the key cell Pt|H₂|H₂O|HgO|Hg|Pt are reported, and the guidelines for further development are outlined.