An electrochemical study using d.c. polarography, cyclic voltammetry and coulometry of o-nitrophenol has been performed in acetonitrile, in the presence of five buffer solutions of pH 20.0, 17.2, 16.2, 11.8 and 8.1. Using linear sweep voltammetry, it was found that at pH 20, the mechanism involves a reversible monoelectronic charge transfer in an E-r, mechanism. This is possible by avoiding the self-protonation reaction. At lower pH values, the mechanism involves a reversible monoelectronic charge transfer followed by protonation steps and homogeneous charge transfer due to disproportionation of the protonated intermediates. At pH 17.2 and 11.8, the mechanisms for the homogeneous charge transfer steps were found to be of DISP2 type (disproportionation order two). At pH 16.2 and 8.1, mechanisms of the DISP1 or ECE type (disproportionation order one) are proposed. Coulometry experiments support the mechanisms proposed : at pH 20, O- phi NO2+1e(-)=O- phi NO2-; at pH 17.2, HO phi NO2+2e(-)+H+=HO phi NO2H-; at pH 16.2, HO phi NO2+2e(-)+2H(+)=HO phi NO+H2O; at pH 11.8, HO phi NO2+3e(-)+3H(+)=HO phi NOH.+H2O; and at pH 8.1, HO phi NO2+4e(-)+4H(+)=HO phi NHOH+H2O. From polarographic I/I-0=f(pH) plots, it was possible to assign pK(a) values to the acid-base pairs involved.