Whereas in aqueous solutions the polarographic reduction of most nitrobenzenes occurs in a single four-electron step, in the presence of 20% or 40% dimethylformamide (DMF) separation of two waves can be observed. The decrease of the first reduction step from a four-electron to a one-electron process with increasing pH is attributed to a decrease in the rate of protonation of the radical ArNO2H.. The decrease from a four-electron to a one-electron wave has the shape of a dissociation curve with an inflexion point at pH = pK’. The dependence of the values of pK’ on the electron-withdrawing substituents indicates the predominant role of adsorption on the rate of protonation of the radical ArNO2H.. The effects of a change in concentration of the co-solvent (DMF) on the values of pK’ are attributed to changes in the solvation energies of ArNO2H2+ and ArNO2H.. The ratio of currents of the reduction of the nitro group in the forward sweep in cyclic voltammetry to those of the oxidation of the resulting arylhydroxylamine on the reverse sweep and to the peak current of the reduction of the nitro group on the second sweep indicate deactivation of the arylhydroxylamine at the electrode surface. The effects of electron-withdrawing substituents on such current ratios indicate the role of a surface reaction of arylhydroxylamine with an intermediate of the reduction of the nitro group, probably ArNO2H..