The effect of different concentrations of the supporting electrolyte tetraethylammonium perchlorate on the standard electrode reaction rate constants for the one-electron reduction of Eu(CF3SO3)(3) on mercury in dimethylsulfoxide has been measured. Two techniques: (i) ac polarography on the dropping mercury electrode; and (ii) cyclic voltammetry (CV) in a two electrode system were employed. The ac data were evaluated both by the Randles and by the Sluyters methods. The standard reaction rates from CV were extracted via a simulation program. Viscosities, resistivities, densities and the double layer capacitances of the electrolytes are given. Correlations between electrolyte properties and the k(s)(app)-values are discussed. The standard electrode reaction rates were analyzed according to the Frumkin theory.