The effect of surface active agents on the thermodynamics and kinetics of electrochemical nucleation has been analysed on the basis of the classical and atomistic theories of phase formation. The theoretical predictions were compared with experimental results obtained by studying the nucleation kinetics of mercury on platinum in both the absence and the presence of potassium nitrate, which proved to be a surface active agent with respect to the bulk mercury phase. The overpotential dependence of the stationary nucleation rate was measured at different potassium nitrate concentrations. It was shown that surface active agents which change the properties of the new bulk phase may not significantly affect the state of small clusters during the process of nucleus formation.