The oxidation-reduction behaviour of 2,2'-dinitrobiphenyl-X type compounds (X = 0, S, NH, CH,, CO) and of 2,2'-dinitrobiphenyl in non-aqueous acetonitrile and dimethylformamide and their mixtures with water was investigated by polarography and cyclic voltammetry at mercury drop electrodes. In combination with ESR-spectroscopy the reversible formation of a radical anion has been found in the primary one-electron step which is followed by a second one-electron uptake leading to a d&anion. According to the group X, to the water content and to the supporting electrolyte the second reduction step is or is not reversible. Theease of reduction decreases in the following sequence with changing X : CO L=-NH z S > 0 > -> CH, (ie E,,, becomes more negative). According to the composition of the solution furtha 2 or 6 electrons are consumed at more negative potentials, giving rise to the 2-hydroxylamine-2'-nitro derivative or to the 2,2'-di-hydroxylamine compound, respectively. In solutions with more than approx. 5-10°/0 (by vol.) of water the reduction of the two nitro groups proceeds either in a single S-electron step or in two Qelectron waves. In 90-94% acetonitrile solutions with alkali metal cation salts as supporting electrolyte a dip appears on the limiting current of the overall S-electron reduction wave with the dme. The inhibition of the reduction process has been interpreted in terms of formation at the electrode of an insoluble precipitate of tri-ions resulting from the interaction between the di-anion and the alkali metal cations.