The streaming mercury electrode was found to be particularly useful for the study of the nonlinear dynamical phenomena (oscillations and multistability) in the electroreduction of pseudohalogenide complexes of nickel(11). Following our earlier experimental and simulation studies, we present the basic theoretical description of such instabilities. First, the steady states were found from the theory of electrode processes controlled by the rate of charge-transfer step, and the stability of these states was determined for the case when the electrode potential E was the only dynamical variable considered. The corresponding instability criterion was derived, which emphasized the role of the capacitive current, permanently charging the surface of the flowing mercury stream. For a further description of both bistability and oscillations in a two-dimensional variable space (electrode potential E -surface concentration of the reactant c(s)) the existence of steady states was analyzed in, terms of the dE/dt and dc(s)/dt nullclines and the bifurcation diagram in the parameter space: (external voltage U - serial ohmic resistance R-s) was constructed. The dimensionless forms of derived equations were briefly discussed in terms of the different time scales for the dE/dt and dc(s)/dt dynamics, what affects the shape of oscillations. Theoretical results were compared with the experimental data for the Ni(Il)-SCN- electroreduction.